Forbid accessing this.args in class-body space

[NullVoxPopuli]

bad:

class extends Component {
  // non-reactive!
  foo = this.args.foo;
}

good:

class extends Component {
  // lazy, reactive
  get foo() {
    return this.args.foo;
  }
}

[wagenet]

Seems reasonable to me. Might want to discuss a bit more if we want it on by default.