Improve help API (#1229)

* WIP

* Fixing all tests

* Removal of old dead code.

* More clean up of help tests

* Fix #987

* Fix #984

* Help builder customizations 

Fixing sizing of grid
Cleaning up some of the APIs

* Adding HelpItem and allowing for getting the contents of help

* Cleaning up the customization

* Fixing max width to be a help builder parameter

* HelpBuilder cleanup

* Adding CommandLineBuilder extensions for configuring help

* Remove unused fields

* Moving [options] before arguments

* Cleaning up naming of descriptor.

Author: Keboo

src/System.CommandLine.DragonFruit.Tests/CommandLineTests.cs

@@ -60,7 +60,7 @@ public async Task It_shows_help_text_based_on_XML_documentation_comments()
             var stdOut = _terminal.Out.ToString();
 
             stdOut.Should()
-                  .Contain("<args>    These are arguments")
+                  .Contain("<args>  These are arguments")
                   .And.Contain("Arguments:");
             stdOut.Should()
                   .ContainAll("--name <name>", "Specifies the name option")
@@ -84,7 +84,7 @@ public void It_synchronously_shows_help_text_based_on_XML_documentation_comments
             var stdOut = _terminal.Out.ToString();
 
             stdOut.Should()
-                .Contain("<args>    These are arguments")
+                .Contain("<args>  These are arguments")
                 .And.Contain("Arguments:");
             stdOut.Should()
                 .ContainAll("--name <name>","Specifies the name option")

src/System.CommandLine.Rendering/ConsoleRenderer.cs

@@ -76,14 +76,14 @@ private void Render(TextSpan span, Region region = null)
                     BackgroundColorSpan.Reset());
             }
 
-            TextSpanVisitor visitor = null;
-
             if (_mode == OutputMode.Auto)
             {
                 _mode = _terminal?.DetectOutputMode() ??
                         OutputMode.PlainText;
             }
 
+
+            TextSpanVisitor visitor;
             switch (_mode)
             {
                 case OutputMode.NonAnsi:

src/System.CommandLine.Tests/Help/Approvals/HelpBuilderTests.Help_describes_default_values_for_complex_root_command_scenario.approved.txt

@@ -1,24 +1,24 @@
-the-root-command:
+the-root-command
   Test description
 
 Usage:
   the-root-command [options] <the-root-arg-no-description-no-default> [<the-root-arg-no-description-default> <the-root-arg-no-default> [<the-root-arg> [<the-root-arg-enum-default>]]]
 
 Arguments:
   <the-root-arg-no-description-no-default>
-  <the-root-arg-no-description-default>       [default: the-root-arg-no-description-default-value]
-  <the-root-arg-no-default>                   the-root-arg-no-default-description
-  <the-root-arg>                              the-root-arg-description [default: the-root-arg-one-value]
-  <Read|ReadWrite|Write>                      the-root-arg-enum-default-description [default: Read]
+  <the-root-arg-no-description-default>     [default: the-root-arg-no-description-default-value]
+  <the-root-arg-no-default>                 the-root-arg-no-default-description
+  <the-root-arg>                            the-root-arg-description [default: the-root-arg-one-value]
+  <Read|ReadWrite|Write>                    the-root-arg-enum-default-description [default: Read]
 
 Options:
-  -trna, --the-root-option-no-arg (REQUIRED)                                                             the-root-option-no-arg-description
-  -trondda, --the-root-option-no-description-default-arg <the-root-option-no-description-default-arg>    [default: the-root-option--no-description-default-arg-value]
-  -tronda, --the-root-option-no-default-arg <the-root-option-arg-no-default-arg> (REQUIRED)              the-root-option-no-default-description
-  -troda, --the-root-option-default-arg <the-root-option-arg>                                            the-root-option-default-arg-description [default: the-root-option-arg-value]
-  -troea, --the-root-option-enum-arg <Read|ReadWrite|Write>                                              the-root-option-description [default: Read]
-  -trorea, --the-root-option-required-enum-arg <Read|ReadWrite|Write> (REQUIRED)                         the-root-option-description [default: Read]
-  -tromld, --the-root-option-multi-line-description                                                      the-root-option
-                                                                                                         multi-line
-                                                                                                         description
+  -trna, --the-root-option-no-arg (REQUIRED)                                                           the-root-option-no-arg-description
+  -trondda, --the-root-option-no-description-default-arg <the-root-option-no-description-default-arg>  [default: the-root-option--no-description-default-arg-value]
+  -tronda, --the-root-option-no-default-arg <the-root-option-arg-no-default-arg> (REQUIRED)            the-root-option-no-default-description
+  -troda, --the-root-option-default-arg <the-root-option-arg>                                          the-root-option-default-arg-description [default: the-root-option-arg-value]
+  -troea, --the-root-option-enum-arg <Read|ReadWrite|Write>                                            the-root-option-description [default: Read]
+  -trorea, --the-root-option-required-enum-arg <Read|ReadWrite|Write> (REQUIRED)                       the-root-option-description [default: Read]
+  -tromld, --the-root-option-multi-line-description                                                    the-root-option
+                                                                                                       multi-line
+                                                                                                       description