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).
4726f30bd6