Add line endings indicator in status bar (#39609)

Closes #5294

This PR adds a line ending indicator to the status bar, hidden by
default as discussed in
https://github.com/zed-industries/zed/issues/5294.

### Changes

- 8b063a22d8700bed9c93989b9e0f6a064b2e86cf add the indicator and
`status_bar.line_endings_button` setting.

- ~~9926237b709dd4e25ce58d558fd385d63b405f3b changes
`status_bar.line_endings_button` from a boolean to an enum:~~
  <details> <summary> show details </summary>

   - `always`     Always show line endings indicator.
- `non_native` Indicate when line endings do not match the current
platform.
   - `lf_only`    Indicate when using unix-style (LF) line endings only.
- `crlf_only` Indicate when using windows-style (CRLF) line endings
only.
   - `never`      Do not show line endings indicator.
   
I know this many options might be overdoing it, but I was torn between
the pleasant default of `non_native` and the simplicity of `lf_only` /
`crlf_only`.

My thinking was if one is developing on a project which exclusively uses
one line-ending style or the other, it would be nice to be able to
configure no-indicator-in-the-happy-case behavior regardless of the
platform zed is running on. But I'm not really familiar with any
projects that use exclusively CRLF line endings in practice. Is this a
scenario worth supporting or just something I dreamed up?

   </details>

- 01174191e4cf337069e7a31b0f0432ae94c52515 rename the action context for
`line ending: Toggle` -> `line ending selector: Toggle`.
When running the action in the command palette with the old name I felt
surprised to be greeted with an additional menu, with the new name it
feels more predictable (plus now it matches
`language_selector::Toggle`!)

### Future work

Hidden status bar items still get padding, creating inconsistent spacing
(and it kind of stands out where I placed the line-endings button):

<img alt="the gap after the indicator is larger than for other buttons"
src="https://github.com/user-attachments/assets/24a346d4-3ff6-4f7f-bd87-64d453c2441a"
/>

I started a new follow-up PR to address that:
https://github.com/zed-industries/zed/pull/39992

Release Notes:

- Added line ending indicator to the status bar (disabled by default;
enabled by setting `status_bar.line_endings_button` to `true`)

docs/src/visual-customization.md

@@ -319,6 +319,10 @@ TBD: Centered layout related settings
     // Clicking the button brings up an input for jumping to a line and column.
     // Defaults to true.
     "cursor_position_button": true,
+    // Show/hide a button that displays the buffer's line-ending mode.
+    // Clicking the button brings up the line-ending selector.
+    // Defaults to false.
+    "line_endings_button": false
   },
   "global_lsp_settings": {
     // Show/hide the LSP button in the status bar.