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).

assets/keymaps/vim.json

@@ -511,7 +511,8 @@
       "}": "vim::CurlyBrackets",
       "shift-b": "vim::CurlyBrackets",
       "<": "vim::AngleBrackets",
-      ">": "vim::AngleBrackets"
+      ">": "vim::AngleBrackets",
+      "a": "vim::Argument"
     }
   },
   {