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.

[ConradIrwin]

Amazing, thank you for this! I think the overall structure is reasonable, and I like that we're doing this as part of detect language.

Looking through the code I have a few suggestions:

• Let's hard limit the number of bytes to search for modeline to ~1k, we've had persistent problems with performance of files with extremely long lines (we could maybe do the emacs thing and make it ~3k, but seems a bit excessive). If we do that then I don't think we need the setting for number of lines.
• We should store the ModelineSettings as an Rc in the Buffer and BufferSnapshot to make it cheaper to create a new snapshot (another area of performance sensitivity for us).
• We need some docs! Maybe a new page describing what variables can be set that's linked to from the existing settings pages?
• Let's (probably) not ship the "emacs locals" syntax. I've seen modelines before, but never that, and don't want to maintain a third parser for this stuff.
• I'd also like to tighten up the syntax for vim. Not sure what's actually used in the wild but in my experience it's always " vim?:. I don't think we need to support ex:`, nor doing this on lines with no comments (and particularly not lines with other non-modeling content before the modeline!).

Thanks again, and let me know if you want help with any of these: https://cal.com/conradirwin/pairing

[ConradIrwin]

(Also feel free to edit typos.toml to fix the style check)