[breaking] only allow enhance on form; pass abort controller

[dummdidumm]

This is a breaking change because the element property was removed from enhance and the action only is possible to apply to forms again; also the interface generic has been removed accordingly.

Fixes #6660
Superseedes/closes #6658
Reverts #6633, because we can achieve the same with event.submitter

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[dummdidumm]

Questioned my sanity until I discovered that event.submitter always seems to have formAction set - and to the default action if you don't have set it which causes subtle bugs. My idea is to check the formAction and infer if it is the default value and if the form has a non-default value then take the form's one, because we can't have both a default and a named action. The check also needs to take into account that actions could point to different URLs

[dummdidumm]

Ok so fixed the issue by applying the logic I described above. The abort controller behavior now is that it not invokes the callback return function, which seems like the best thing to do. I didn't want to introduce another { type: 'aborted' }, which you could also make an argument for.