Implement color spaces

[nex3]

• Implementation (Language)
• Implementation (JS API)
• Breaking changes

See sass/sass#2831

[stof]

@nex3 what is the state of this implementation ? Color Level 4 is now coming to all browsers (shipped in Safari and Chrome; available behind a flag in Firefox which is enabled by default in Firefox Nightly so likely to ship in a few weeks) so it would be great to have access to them in Sass.

[nex3]

@mirisuzanne and I are still hammering out the details. A draft implementation is available on the features.color-4 branch, but the spec keeps changing out from under us so it's been difficult to nail down 100% final behavior. (It's paradoxically easier for browsers to change their edge case behavior than for us, since browsers have the ability to see whether something is used in the wild.)

The other big blocker here is getting tests written/updated for every corner of the (very large) proposal. Although the implementation is likely mostly correct, I'm sure I got some corners wrong in ways we won't know until we test it.

[stof]

@nex3 the implementation should be marked as done