Add an EditorConfig file (#600)

Add an EditorConfig (`.editorconfig`) file to this repo.

### Motivation:

[EditorConfig](https://editorconfig.org) helps maintain consistent
coding styles for multiple developers working on the same project across
various editors and IDEs. It will be helpful to add one of these files
to swift-testing so that all contributors use the whitespace conventions
described in our [style
guide](https://github.com/swiftlang/swift-testing/blob/main/Documentation/StyleGuide.md)
automatically when using a supported IDE/editor.

Note that Xcode 16 Beta has added support for EditorConfig files — see
20796230 in the [release
notes](https://developer.apple.com/documentation/xcode-release-notes/xcode-16-release-notes).

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

.editorconfig

@@ -0,0 +1,11 @@
+# EditorConfig documentation: https://editorconfig.org
+
+root = true
+
+# Default settings applied to every file type.
+[*]
+indent_style = space
+indent_size = 2
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true