Mark many methods as @internal

[oliverklee]

I propose we mark public methods which are not expected to be used when an application parses CSS and uses the result as @internal in order to communicate that these methods may change between releases without deprecation and without a major version change (i.e., in a release that is not expected to have breaking changes according to semantic versioning).

This would allow us to boldy refactor this library without bad surprises for users.

@sabberworm Would this approach be okay for you?