Combined Class Fields implementation and spec updates

Developers of JavaScript want to be able to test out new features to provide feedback to specification drafts, as well as making their code more concise and readable. Implementing private fields as part of the Class Fields proposal will allow these people to encapsulate code they don't want to expose as part of their API.

This will be implemented as Babel transform plugin, which will also involve changes to Babylon (the parser used by Babel). My work will also extend to updating existing plugins to updated specification drafts, as well as fixing bugs in existing code.