DEV: makes transformComplete async (PR #12752)

This is used to apply multiple transformations following user input.

GitHub

Fingers crossed! :grinning_face_with_smiling_eyes:

Note this is used for the experimental: GitHub - discourse/bbcode-tag-autocomplete and will also need DEV: adds an experimental `preserveKey` bool option to autocomplete by jjaffeux · Pull Request #12751 · discourse/discourse · GitHub