xfd: when cfd + rename, warn or abort if proposed rename already exists

[NovemLinguae]

Requested by Certes at https://en.wikipedia.org/w/index.php?title=Wikipedia_talk:Twinkle#Rename_category

Diffs: https://en.wikipedia.org/w/index.php?title=Special:Contributions&offset=20220309145604&target=Certes&limit=4

Could either check and display a warning somewhere on the form after the target is typed in, or could abort during the submit process.