[TASK] Mark OutputFormatter as @internal

[oliverklee]

This class should only be used for formatting CSS from within this library. It is not intended to be called from outside.

[coveralls]

coverage: 55.726%. remained the same
when pulling 900883c on task/internal-formatter
into 8dcc1a5 on main.

[JakeQZ]

I think so.

@sabberworm, would you agree?

[JakeQZ]

Actually maybe not. See #895 (comment).

It might not be used from the outside, but may need to be constructed and passed around, depending on how we resolve the circular dependency.

[oliverklee]

I've proposed a path forward in #907.

[oliverklee]

@sabberworm ping

[JakeQZ]

Marking as draft pending #907,

[oliverklee]

Ping @JakeQZ @sabberworm - I'd like to get this merged soon-ish so I can reduce the number of open PRs created by me (which reduces the amount of rebasing I need to do).

[JakeQZ]

Yes, I think currently users of this library provide an OutputFormat method which internally has a reference to an OutputFormatter which does the work and maintains state. I guess it's analogous to a Car and an Engine. The car has an engine rather than the other way round, and the user drives the car without needing to touch the engine directly. The same car can visit the various CSS elements.

Also, this is consistent with the proposed change in #898. So 👍 from me now.