Add vim/emacs modeline support

[elmarco]

Many editors such as vim and emacs support "modelines", a comment at the beginning of the file that allows the file type to be explicitly specified along with per-file specific settings

• The amount of configurations, style and settings mapping cannot be handled in one go, so this opens up a lot of potential improvements.
• I left out the possiblity to have "zed" specific modelines for now, but this could be potentially interesting.
• Mapping the mode or filetype to zed language names isn't obvious either. We may want to make it configurable.

This is my first contribution to zed, be kind. I struggled a bit to find the right place to add those settings. I use a similar approach as done with editorconfig (merge_with_editorconfig). There might be better ways.

Closes #4762

Release Notes:

• Add basic emacs/vim modeline support.

Supersedes #41899, changes:

• limit reading to the first and last 1kb
• add documentation
• more variables handled
• add Arc around ModelineSettings to avoid extra cloning
• changed the way mode -> language mapping is done, thanks to modeline_aliases language config
• made "Local Variables:" handling a separate commit, so we can drop it easily
• various code style improvements