[TASK] Separate MM_match_fields from MM_insert_fields handling

RelationHandler property $this->MM_insert_fields state
falls back to MM_match_fields in start(). This is ugly
and can be easily separated by adapting the usage
in writeMM() a bit. This avoids magic and reduces
unexpected side effects.

Change-Id: Ie8c84bbbb04544844beeb71240da45a6bb9ed2ab
Resolves: #100256
Releases: main
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78213


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>