(PSI): introduce trait_body

[mkaput]

No description provided.

[matklad]

Hm, looks like in the go plugin they do not introduce a special syntactic category for bodies of things. https://github.com/go-lang-plugin-org/go-lang-idea-plugin/blob/58a85af0be6652f1882ba5725588d691af7cd27d/grammars/go.bnf#L168

I guess this will not work for us, because we have rather more involved headers.

[mkaput]

I find multiple advantages of this solution:

1. I believe this will make creating indentation fixer easier
2. Expand selection <3
3. Kotlin & Java do this

Disadvantage: **Body.foo instead of just foo