What:
Add a hook that runs before a save attempt is made in
ApiVisualEditorEdit. The hook receives the same data available in
ApiVisualEditorEdit, and implementations of the hook can modify the API
response.
Why:
VE plugins may send additional data when saving an edit, and extensions
might want to prevent the save from taking place based on that
additional data.
See for example the AddLink plugin in Ic8225933c9, where the save is
blocked if link suggestions don't exist in the database at save time.
Bug: T283109
Change-Id: I6b81ea318f52ec47661086d85b5cc242a3fcd0e4
