Enhances `sqlfmt` support in SQL layer.
`sqlfmt` was only being applied against the whole buffer, and did not allow
invocation against a specific region in the buffer. `sqlfmt` customize options
were not included under the `sql` group. It was not possible to control if
`sqlfmt` reused an existing error buffer or generated new ones.
- Added `sqlfmt-region` interactive command and binding under `sql-mode` keymap
- `sqlfmt-buffer` calls into `sqlfmt-region` with `(point-min)` and
`(point-max`)
- Added `sqlfmt` customize group as child to `sql` customize group
- Added `sqlfmt-reuse-error-buffer` option
- Updated `sql` layer documentation to reflect changes
