Allow authoring and fix abstract slot inference

[dsyme]

1. Allow authoring of static abstracts in interfaces

2. Apply abstract slot inference

Tested manually, it's OK to merge this into the PR I think

TODOs to add

• Check that we can't author static abstracts in classes or structs
• Add tests for authoring static abstracts in interfaces
• Add tests for abstract slot inference

Tested manually on

type IAdditionOperator<'T> =
    static abstract op_Addition: 'T * 'T -> 'T

type C() =
    interface IAdditionOperator<C> with
        static member op_Addition(x, y) = C()

and then by inspecting generated IL.

[dsyme]

I'm getting

        "Feature 'static abstract interface members' is not supported by target runtime." }]

for all the tests - how do we run them on .NET 7?