Prototype assignments count as method-like

[sandersn]

For the purposes of reporting prototype/instance property conflicts, the compiler previously assumed that only methods were prototype properties. The intent of the error is to prevent instance vs property confusion like this:

class Super {
  constructor() {
    this.p = () => 1
  }
}
class Sub extends Super {
  p() { return 2 }
}
console.log(new Sub().p()) // 1 ???!!!? i warned you about classes bro!!!! i told you dog!

class Base {
  get x() { return 1 }
}
class Derived extends Base { }
Derived.prototype.x = 2
console.log(new Derived().x) // 1 ??/?? it keeps happening

However, in Javascript, any assignment directly to the prototype is a prototype property, as in the line Derived.prototype.x = 2. So there's no conflict when the base property is on the prototype and the derived one is too:

class Base { }
Base.prototype.mustImplement = null
class Derived extends Base {
  mustImplement() {
    return 101
  }
}

This abstract-like pattern appears in webpack. Note that "isMethodLike" should really be "isPrototypeProperty" but the compiler is not very good at tracking this distinction. I optimistically renamed the function and documented its limitations in a comment.

Fixes #22895 (for real this time)