Skip non-BMP bidi mirroring pairs instead of failing

Unicode 18 added mirroring pairs involving supplementary code points
(e.g., 221D;1DB10), which made the generator raise and broke make unicode.
iTermBidiMirroredCounterpart maps a unichar to a unichar, so such pairs
can't be applied; omit them in both directions so the table stays an
involution.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>