[ShadowDOM] Implement moveBefore() #631
Description
Description
moveBefore() is a new API similar to insertBefore(). I won't be possible to polyfill, but ShadyDOM could possibly wrap it when available to create the right DOM structure.
A good addition here would be to only patch moveBefore() when it's available natively so that feature detection in library code still works.
Motivation
When working on moveBefore() support for lit-html, I hit an error with the polyfill tests: lit/lit#4838
I haven't made a minimal reproduction yet, but the native function is complaining about an invalid hierarchy.