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).
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