aliasing -webkit-clip-path to clip-path

[karlcow]

@wisniewskit just added an alias for -webkit-clip-path to clip-path for Firefox
see https://bugzilla.mozilla.org/show_bug.cgi?id=1800969

It might be worth considering adding it to the spec
https://compat.spec.whatwg.org/#css-simple-aliases

The CSS spec doesn't mention any alias so far
https://drafts.fxtf.org/css-masking/#the-clip-path

[miketaylr]

I'm surprised they shipped that without updating the spec (this one, or in css-masking). @wisniewskit - want to send in a PR? 😄