RFC: LAB -> Lab, LUV -> Luv

This renames `LAB` to `Lab` and `LUV` to `Luv` but adds typealiases for backwards compatibility. @glenn-sweeney pointed out the inconsistency and it's bothered me every since. :wink:

I'm leaving the parameters all lowercase though, since unlike `xyY`, case isn't needed to distinguish parameters.

Anyone object?