Should reject changing method visibility while overriding

[rix0rrr]

The following is NOT allowed in C#:

class A {
    protected fn() {
    }
}

class B extends A {
    public fn() {
    }
}

I honestly don't understand why, it seems like yet another perfectly reasonable thing to allow, but like Return Value Covariance it's one of those things that C# draws a hard line on and so we have to disallow it in jsii.

[RomainMuller]

Closed by #1876