Css preprocessing

[Rich-Harris]

This builds atop #959 — a few tweaks here and there, but the biggest change is that it removes Pug, CoffeeScript etc, as it's not necessary to include those in order to test preprocess

[codecov-io]

Codecov Report

Merging #969 into master will increase coverage by 0.02%.
The diff coverage is 92.59%.

@@            Coverage Diff             @@
##           master     #969      +/-   ##
==========================================
+ Coverage   91.76%   91.78%   +0.02%     
==========================================
  Files         109      110       +1     
  Lines        4054     3980      -74     
  Branches     1301     1304       +3     
==========================================
- Hits         3720     3653      -67     
+ Misses        150      142       -8     
- Partials      184      185       +1

Impacted Files	Coverage Δ
src/index.ts	86.2% <92.59%> (+4.95%)	⬆️
test/parser/index.js	100% <0%> (ø)
src/shared/utils.js	100% <0%> (+8.41%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 72bd23a...b856ac2. Read the comment docs.

[esarbanis]

Oh, wow! I sure learnt a lot from this PR. Thanks for your time!

[Rich-Harris]

No, thank you! It's great to finally have a solution to this problem. Release coming soon