Implement setPublicClassFields and privateFieldsAsProperties assumptions

[nicolo-ribaudo]

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

Main PR: #12219
RFC: babel/rfcs#5

I also introduced a warning when both loose and setPublicClassFields/privateFieldsAsProperties are set (while still maintianing the fallback behavior we have for the other assumptions).
This is because loose support for these class features plugins is a mess and easily breaks: it was what initially lead me to the assumptions RFC. We should encourage users to fully migrate to assumptions for these plugins when they start using them, to avoid other possible configuration conflicts caused by the implicit dependencies between them.

The tests are what we already had for loose.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/37890/

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[JLHwung]

This will throw on @babel/core 7.12 + @babel/plugin-class-properties 7.13 because api is defined but api.assumption is not provided.
Never mind, I always forget the fact that api.assumption has been provided by helper plugin utils.

CI error is related.

[nicolo-ribaudo]

The CI failure caught an assumption I didn't notice while writing the RFC.

The super binding of a class instance is mutable:

class B {
  method() { alert("B!"); }
}

class C {
  method() { alert("C!"); }
}

class A extends B {
  run() { super.method(); }
}

Object.setPrototypeOf(A.prototype, C.prototype);

new A().run();

For this reason, we compile super.method to _get(_getPrototypeOf(A.prototype), "method", this).

However, in loose mode we assume that it's never mutated and thus we compile it to _B.prototype.method.

I'm going to add this to the RFC.

[JLHwung]

Consider change the signature to { privateFieldsAsProperties: boolean } so we can pass noDocumentAll later. This function is not a public interface so it should be okay.