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.

[lcnittl]

I think that this is possible by adding your desired language to sqlfluff.format.languages, and then (after reloading VS Code once) selecting the SQL string and running SQLFluff Format Selection from the command palette.