Intent to prototype and ship: -webkit-image-set() alias to image-set()

[Emilio Cobos Álvarez]

Hi, per discussion in:

* https://github.com/whatwg/compat/issues/144
* https://github.com/w3c/csswg-drafts/issues/6285

There's enough usage of -webkit-image-set on the wild that it might be
worth supporting this as an alias of the new syntax (which is a
super-set of what Chrome and WebKit implement with the prefixed syntax)
to improve the experience of Firefox users in HiDPI displays.

WebKit in fact already does this to some extent (they parse
-webkit-image-set(), but serialize it as image-set()).

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1709415

Standard: CSS resolution in https://github.com/w3c/csswg-drafts/issues/6285

Platform coverage: All

Preference: None

DevTools bug: N/A

Other browsers: Both WebKit and Blink ship this as you could imagine.
WebKit does implement the unprefixed version of the function as
described above.

web-platform-tests: Added as part of my Gecko patch.

Let me know if there's any concern with this.

-- Emilio