#22862: Consider a CSS preprocessor

In #22862, it was proposed that core move to using a CSS preprocessor. Given that we now have the appropriate tooling in place and MP6 has been greenlighted for preparation to merge into core, it is time we hash this out: which, if any, CSS preprocessor should we use, why that particular one, what does implementing it take, and what does the contribution process look like? Some of this discussion has already taken place on-ticket, which is great, including a ton of proof-of-concept patching by @wonderboymusic.

For further reference, MP6 currently uses SCSS for its color schemes, with a branch that demonstrates what it would look like in LESS. Many thanks to @ryelle for maintaining the LESS branch. By no means does this dictate what happens in core itself, but it is certainly an influence and a great data point.