Add a command to expand the context for a multibuffer (#10300)

This PR adds an action to expand the excerpts lines of context in a
multibuffer.

Release Notes:

- Added an `editor::ExpandExcerpts` action (bound to `shift-enter` by
default), which can expand the excerpt the cursor is currently in by 3
lines. You can customize the number of lines by rebinding this action
like so:

```json5
// In your keybindings array...
  {
    "context": "Editor && mode == full",
    "bindings": {
      "shift-enter": ["editor::ExpandExcerpts", { "lines": 5 }],
    }
  }
```

---------

Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Max <max@zed.dev>