Handle the command selector an IME hands back after committing. Issue 13030

When an input method finishes a composition by committing text via
insertText:replacementRange: and then hands the original keystroke
back as a command selector (deleteBackward: or insertNewline:),
iTerm2 dropped the selector. With the Korean 2-Set IME this left a
stray jamo on Backspace and swallowed Return during composition.

Now doCommandBySelector: sends the event to the delegate when the IME
committed text during the same keypress. Verified with Korean 2-Set,
Japanese Hiragana, and Chinese Pinyin input sources: the Japanese and
Chinese IMEs consume Return themselves when confirming a candidate,
so they are unaffected.

Based on GitHub PR 752 by Sejin Hong.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>