This PR updates the custom styles API, adding toExternalHTML and parse. The motivation behind it is mainly to fix external HTML export for text/background color styles (see more about this in #1991).

TODO:

• Fix TipTap mark internal representation in styleSpecs. We need to call the mark's renderHTML function in styleSpec.implementation.render, but it's not getting all the arguments it needs atm and throwing errors.
• Fix typing.
• Add changes to React API.
• Updates snaps & check for expected output