Add new argument vim text object (#7791)

This PR adds a new `argument` vim text object, inspired by
[targets.vim](https://github.com/wellle/targets.vim).

As it's the first vim text object to use the syntax tree, it needed to
operate on the `Buffer` level, not the `MultiBuffer` level, then map the
buffer coordinates to `DisplayPoint` as necessary.

This required two main changes:
1. `innermost_enclosing_bracket_ranges` and `enclosing_bracket_ranges`
were moved into `Buffer`. The `MultiBuffer` implementations were updated
to map to/from these.
2. `MultiBuffer::excerpt_containing` was made public, returning a new
`MultiBufferExcerpt` type that contains a reference to the excerpt and
methods for mapping to/from `Buffer` and `MultiBuffer` offsets and
ranges.

Release Notes:
- Added new `argument` vim text object, inspired by
[targets.vim](https://github.com/wellle/targets.vim).

crates/editor/src/highlight_matching_bracket.rs

@@ -17,7 +17,7 @@ pub fn refresh_matching_bracket_highlights(editor: &mut Editor, cx: &mut ViewCon
     let snapshot = editor.snapshot(cx);
     if let Some((opening_range, closing_range)) = snapshot
         .buffer_snapshot
-        .innermost_enclosing_bracket_ranges(head..head)
+        .innermost_enclosing_bracket_ranges(head..head, None)
     {
         editor.highlight_background::<MatchingBracketHighlight>(
             vec![