Standardize on CursorShape::Underline not Underscore (#19028)

Currently terminal.cursor_shape uses `underline` and `cursor_shape` uses
`underscore`.
This standardizes them so they use the same settings value.

I think `underline` is the more common term and it matches the
terminology used by VSCode, Alacritty, iTerm, etc.

Note the protobuf enum `CursorShape::CursorUnderscore` remains
unchanged.

See also:
- https://github.com/zed-industries/zed/pull/18530
- https://github.com/zed-industries/zed/pull/17572

Release Notes:

- Settings: rename one `cursor_shape` from `underscore` to `underline`
(breaking change).

docs/src/configuring-zed.md

@@ -371,10 +371,10 @@ List of `string` values
 "cursor_shape": "block"
 ```
 
-3. An underscore that runs along the following character:
+3. An underline / underscore that runs along the following character:
 
 ```json
-"cursor_shape": "underscore"
+"cursor_shape": "underline"
 ```
 
 4. An box drawn around the following character: