Fix indent/dedent for "return" syntax. #6564
Description
While working on #6333, the dedent regex was kept simple for the return keyword (see #6497 (comment)), but didn't cover the case for return followed by a dangling [, ( or { before the cursor.
This issue also covers restoring the dedent removed for #6813 (added for #6333).
Current (pre-#6813) behaviour (merged in #6497):
Expected/desired behaviour:
