Add vim/emacs modeline support (#44210)

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
- drop vim ex: support
- made "Local Variables:" handling a separate commit, so we can drop it
easily
- various code style improvements

---------

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/buffer_diff/src/buffer_diff.rs

@@ -2,8 +2,8 @@ use futures::channel::oneshot;
 use git2::{DiffLineType as GitDiffLineType, DiffOptions as GitOptions, Patch as GitPatch};
 use gpui::{App, AppContext as _, Context, Entity, EventEmitter, Task};
 use language::{
-    Capability, Diff, DiffOptions, File, Language, LanguageName, LanguageRegistry,
-    language_settings::language_settings, word_diff_ranges,
+    Capability, Diff, DiffOptions, Language, LanguageName, LanguageRegistry,
+    language_settings::LanguageSettings, word_diff_ranges,
 };
 use rope::Rope;
 use std::{cmp::Ordering, future::Future, iter, ops::Range, sync::Arc};
@@ -946,7 +946,6 @@ impl BufferDiffInner<language::BufferSnapshot> {
 }
 
 fn build_diff_options(
-    file: Option<&Arc<dyn File>>,
     language: Option<LanguageName>,
     language_scope: Option<language::LanguageScope>,
     cx: &App,
@@ -962,7 +961,7 @@ fn build_diff_options(
         }
     }
 
-    language_settings(language, file, cx)
+    LanguageSettings::resolve(None, language.as_ref(), cx)
         .word_diff_enabled
         .then_some(DiffOptions {
             language_scope,
@@ -1494,7 +1493,6 @@ impl BufferDiff {
         let base_text_changed = base_text_change.is_some();
         let compute_base_text_edits = base_text_change == Some(true);
         let diff_options = build_diff_options(
-            None,
             language.as_ref().map(|l| l.name()),
             language.as_ref().map(|l| l.default_scope()),
             cx,