[static private] Unify loose handling of static and instance props

[nicolo-ribaudo]

Q	A
Fixed Issues?	y
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

Follow up to #8205

Since both static and instance private props are defined a non-configurable non-enumerable properties on their target, thay can be handled in the same way.

This PR also removes a possible conflict between private and public static properties, by using the classPrivateFieldLooseKey helper to generate the key (instead of "_" + name). e.g.

class Foo {
  static #bar = 1;
  static _bar = 2;
}

cc @rricard @jridgewell

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/9007/

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/8995/