Separate `SetAttr`, `SetItem`, `SetVar` in AST

[MarcellPerger1]

Maybe separate SetAttr, SetItem, SetVar in AST could be useful (instead of general AstAssign).
There is often completely different functionality for these (e.g. in type checking, code gen).
(Although Python doesn't separate it for a reason?)

• Decide whether to do this at all
• Add the classes
• Update AstGen
• Update snapshot tests
• Update typechecker (if done by then)