More systematic completion for all kinds of case labels

#1312 tries to fix a regression in completion after `case`, which was caused by recent work concerning Java 17.

Looking at the code changes, however, I don't get a clear picture of the various styles of `case` and what kinds of completions should be proposed for each.

Hence I would like to see this area captured more systematically, before we continue tweaking issues back and forth.

Start by reviewing issue #1312 and the recent Java 17-related completion changes. Compare the different styles of case labels and define the expected completion proposals for each; done means the behavior is captured systematically before further tweaks are made.

Written by the indexing model from the issue text.