Make @babel/plugin-class-features a normal helper package

[nicolo-ribaudo]

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

This is a better alternative to #9050.

---

This PR removes the @babel/plugin-class-features package, which becames @babel/helper-create-class-features-plugin and can only be used inside other plugins.

The problem with the @babel/plugin-class-features plugin is that, in order to allow using @babel/plugin-proposal-class-properties, etc., it behaves differently when declared multiple times:

{
  "plugins": [
    ["@babel/plugin-class-features", { "fields": true }],
    ["@babel/plugin-class-features", { "decorators": true }]
  ]
}

For any other plugin, the last specified options override the others, but in this case they are merged.

In #9050 I removed direct options to that plugin, but it doesn't make sense to allow using it as plugins: ["@babel/plugin-class-features"] since it does nothing.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/9487/

[Andarist]

maybe @babel/helper-create-class-feature-plugin or @babel/helper-class-feature-factory?

[nicolo-ribaudo]

I like the first name 👍

[existentialism]

@babel/helper-create-class-feature-plugin 👍

[hzoo]

awesome, it's a good idea. It seems weird to have another way to turn on a plugin (otherwise I would move all experimental features into the same plugin). Each plugin is separately turned on in a consistent way this way.

[loganfsmyth]

I think I'd rather keep the names consistent with the other plugins. Maybe we can delete this line and have the plugin do:

return {
  name: "...",
  ...createClassFeaturePlugin({ /* ... */ })
};

?