Add cursor_shape setting for the default editor (#17572)
Closes https://github.com/zed-industries/zed/issues/16451,
https://github.com/zed-industries/zed/issues/14447,
https://github.com/zed-industries/zed/issues/7203
Addresses but does not closes
https://github.com/zed-industries/zed/issues/5179
Expose cursor shape selection to users. Possibly controversial, since
block cursor seems preserved for vim and terminal. But the heart wants
what it wants?
Release Notes:
- Added a setting for `cursor_shape`. Can be `bar`, `block`,
`underline`, or `hollow`. Default is `bar`.