[PATCH] Make it easier to add specially treated relations to SplitWayCommand

[taylor.smock]

This patch should make it easier to add specially treated relations for splitting to its special case.

This was partially influenced by ​https://wiki.openstreetmap.org/wiki/Proposed_features/Connectivity -- it is probably a bad idea to break those relations, if they exist.

The current patch is mostly a refactor. It does not add the Connectivity relation type as a special case.

It adds a new static variable with currently existing special cases.
The static variable is a Map<String, String>, with the key being a restriction type and with the value being the specific case (currently, it should only be "restriction"). I've also moved the restriction specific code to its own method, to increase readability if another special case is found/created.

[taylor.smock]

Partial refactor of SplitWayCommand to allow for easier adding of specially treated relations

[Don-vip]

In 15078/josm:

fix #17718 - Make it easier to add specially treated relations to SplitWayCommand (patch by taylor.smock)