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/vim/src/vim.rs

@@ -545,10 +545,10 @@ impl Vim {
                 if self.operator_stack.is_empty() {
                     CursorShape::Block
                 } else {
-                    CursorShape::Underscore
+                    CursorShape::Underline
                 }
             }
-            Mode::Replace => CursorShape::Underscore,
+            Mode::Replace => CursorShape::Underline,
             Mode::Visual | Mode::VisualLine | Mode::VisualBlock => CursorShape::Block,
             Mode::Insert => CursorShape::Bar,
         }