Fix private getter/setter mangling

[jridgewell]

This fixes an error with private getter/setters being mangled as if they were regular public properties:

export class Foo {
  get #aaaaaa() {}
  constructor() {
    this.#aaaaaa
  }
}

In this, get #aaaaaa {} is a AST_PrivateGetter, which isn't a subclass of AST_ClassPrivateProperty nor AST_PrivateMethod. But it is a subclass of AST_ObjectProperty. So our property mangler was mangling the key as if it were a public property, and not consulting the private_cache mapping. So the mangler renames the getter to #o and the access to #a:

class Foo {
    get #o() {}
    constructor() {
        this.#a;
    }
}

This also enables private property mangling by default whenever we're doing a normal mangle pass. This is as safe as mangling regular const variable_name = 1, because private properties are completely analyzable in the lexical scope.

Re: #1012

[fabiosantoscode]

This seems to close #1012

[jridgewell]

Possibly, I don't know if you want to use #1012 to mention that we do private property mangling as a default mangle step. But yes, this does move the mangling into the default step as requested in #1012.

[fabiosantoscode]

Ok, I checked it more thoroughly, it looks great!

[fabiosantoscode]

Thank you! ❤️