- Introduce a `default_range` field to `VimCommand`, to be optionally
  used when no range is specified for the command
- Update `VimCommand.parse` to take into consideration the
  `default_range`
- Introduce `CommandRange::buffer` to obtain the `CommandRange` which
  corresponds to the whole buffer
- Update the `VimCommand` definitions for both `sort` and `sort i` to
  default to the whole buffer when no range is specified

Closes #41750 

Release Notes:

- Improved vim's `:sort` command to sort the buffer's content when no
selection is used