Pattern accepted by parser but rejected by MRI

[mbj]

In reference to: mbj/unparser#340 (comment), it appears that parser currently accepts syntax that is rejected in MRI:

case foo
in {"#{"a"}": 1}
end

This is accepted in recent parser releases, in ruby version 2.7 to 3.2. Also accepted by MRI 2.7, but rejected 3.0-3.2.

I think we should adapt the parser grammars for 3.0+ to reject this also. Thanks to @kddnewton for reporting in unparser.