Intellisense for overriding methods

Specifically, I would like to see Intellisense enabled when I type
```fs
[<AbstractClass>] type A() = abstract M : unit -> unit
type B() = inherit A()
           override __.
```

A member list with `M` would be shown, and I can select it. This is super useful to find out which methods can be overridden for a class from a library.