Implement LAB/HSL color interpolation for expressions

[mourner]

The expressions PR is already merged into master, but it removed support for LAB/HSL colorSpace property.

We should either add it back before a release (since it's a breaking removal), or remove its support fully (along with color_spaces.js and related code) if we don't plan on also implementing it on the Native side.

cc @anandthakker @kkaefer

[mourner]

Another option would be to roll back expressions-based stop functions conversion until we reach feature and performance parity.

[anandthakker]

Oops, I intended to ticket this follow-up, but forgot to -- thanks @mourner .

@jfirebaugh had an idea about removing the colorSpace property from stop functions, and instead making it part of the *-transition properties.

[mourner]

Hmm, how would that look? trasition properties are for timed fades, while colorSpace controls the way colors are interpolated between stops in a function.

[anandthakker]

trasition properties are for timed fades

Yeah, I think in order to use that approach we'd probably have to broaden the idea of what the transition properties are for. The advantage, though, is that there would be a consistent way to control color interpolation both for DDS and for the case of transitioning between constant paint property values (e.g. via setPaintProperty() / setStyle())

[jfirebaugh]

Retitled/de-labeled, given #5379.