Expose enum variant fields on interface Variant

[staniewzki]

I created this issue to track task assignments.

Original description:
- they are already available on type StructVariant
- the edge on type PlainVariant should exist but never produce any vertices
- the edge on type TupleVariant should produce fields similarly to how tuple structs produce their fields
- new lints enabled by this:
- tuple variant field removed
- exhaustive tuple variant field added