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/terminal/src/terminal_settings.rs

@@ -134,7 +134,7 @@ pub struct TerminalSettingsContent {
     /// Default: {}
     pub env: Option<HashMap<String, String>>,
     /// Default cursor shape for the terminal.
-    /// Can be "bar", "block", "underscore", or "hollow".
+    /// Can be "bar", "block", "underline", or "hollow".
     ///
     /// Default: None
     pub cursor_shape: Option<CursorShape>,