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_view/src/terminal_element.rs

@@ -793,7 +793,7 @@ impl Element for TerminalElement {
                             let (shape, text) = match cursor.shape {
                                 AlacCursorShape::Block if !focused => (CursorShape::Hollow, None),
                                 AlacCursorShape::Block => (CursorShape::Block, Some(cursor_text)),
-                                AlacCursorShape::Underline => (CursorShape::Underscore, None),
+                                AlacCursorShape::Underline => (CursorShape::Underline, None),
                                 AlacCursorShape::Beam => (CursorShape::Bar, None),
                                 AlacCursorShape::HollowBlock => (CursorShape::Hollow, None),
                                 //This case is handled in the if wrapping the whole cursor layout