Add support for `context-fill` and `context-stroke`

[LaurenzV]

Here is the corresponding specification.

This is still a WIP, in particular, the following I still haven't looked into more deeply (as I'm still unfamiliar with the codebase) and I think might be a bit tricky to implement:

When the context paint layers include paint server references, then the coordinate space and the bounding box used to scale the paint server elements and content are those of the context element. In other words, any gradients and patterns referenced with these keywords should be continuous from the main shape to the markers, or from one element within a use-element shadow tree to another.

But I'm opening this as a draft PR so that people know that someone is working on it. 😄

I also had to temporarily switch to my own version of svgtypes since it's not included in this repository.