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).

crates/editor/src/editor_settings.rs

@@ -179,7 +179,7 @@ pub struct EditorSettingsContent {
     /// Default: true
     pub cursor_blink: Option<bool>,
     /// Cursor shape for the default editor.
-    /// Can be "bar", "block", "underscore", or "hollow".
+    /// Can be "bar", "block", "underline", or "hollow".
     ///
     /// Default: None
     pub cursor_shape: Option<CursorShape>,

crates/editor/src/element.rs

@@ -6060,7 +6060,7 @@ impl CursorLayout {
                 origin: self.origin + origin,
                 size: size(self.block_width, self.line_height),
             },
-            CursorShape::Underscore => Bounds {
+            CursorShape::Underline => Bounds {
                 origin: self.origin
                     + origin
                     + gpui::Point::new(Pixels::ZERO, self.line_height - px(2.0)),