Map custom elements to two different features (#46309)
This should match web-platform-dx/web-features#1089.
The test results on wpt.fyi were used as a guide for where there might
be tests involving customized built-in elements, and all tests with
failures were given a quick review before deciding where to map it.
Mistakes are still possible.
This is the most complicated mapping to web-features so far, and shows
some limitations of the WEB_FEATURES.yml format. This could be
simplified with exclusion patterns in the format, but would still not be
simple.
Two files were renamed in the process of preparing this:
#46305
#46306