1) Move paintWorklet from "Window" to "CSS" according to new specification;
2) Enable WPT css-paint-api tests from upstream and remove forked duplicates from mozilla folder
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#19840
- [X] These changes do not require tests because the change was made in tests it-selves
Source-Repo: https://github.com/servo/servo
Source-Revision: a0e340d68f5a51d61a69b0bc16f543d3cef54597