Align implementation with spec of soft modifiers

[odersky]

The spec in https://dotty.epfl.ch/docs/reference/soft-modifier.html says:

A soft modifier is treated as potential modifier of a definition if it is followed by a hard modifier or a keyword combination starting a definition (def, val, var, type, given, class, trait, object, enum, case class, case object). Between the two words there may be a sequence of newline tokens and soft modifiers.

But the implementation recognized also soft modifiers in front of just case, not followed by class or object.
The change caused some breakage for enum cases, where we have case alone, but that one cannot be preceded by any
soft modifiers anyway. So only neg tests were affected.

Fixes #15960

[odersky]

@tgodzik I am not sure about the consequences for Metals. The problem I could see is where we type something like

open case 

and want to continue with class. Will the IDE get confused at this point?

[tgodzik]

From Metals perspective we might need to improve the error in Scalameta, but the presentation compiler features should work with whatever compiler provides us. We don't have any specific rules to completions after open case, but might be good to add a test case for it.