Add MakeDeclarationMutable code fix

[cartermp]

This makes #1110 a bit easier too.

Works where it should work:

And it does not trigger if the value is a parameter binding, which would be invalid F# syntax:

The above shows in the status bar if you try it for code like this:

type C() =
    member _.M(x: int) =
        x <- 1

let f (x: int) =
    x <- 1
    x