feat: Command to format selection instead of whole file

[nikclayton]

Consider a file that's not SQL, but contains embedded SQL, perhaps a multi-line string.

val query = """
SELECT x
      FROM
  bar
"""

The extension can't currently process that.

I'd like to be able to select the SQL (either manually, or, preferably, have it auto-select everything in the string the cursor is in), run the "fix" command, and have the fixed query replace the selection.