use extractCritical to set css-vars inline

[kylpo]

What:
This PR adds css-vars to extractCritical so their initial values may be set inline (when in extract mode). Discussed in #147.

How:
Started with @mitchellhamilton's regex substitute, but needed to account for vars- strings a bit differently. Since the regex was greedy, for a string like

.vars-176qc2r {--css-Home-1wjhkv7-0: 0; --css-Home-1wjhkv7-1: 1}

, the matcher would grab a css- string instead of the desired vars-. We could change our regex to be lazy with something like /(?:css|vars)(?:[a-zA-Z0-9-]*?)-([a-zA-Z0-9]+?)/gm, but that seems brittle. Surely there'll be a time when css- will be shorter than the vars-.

So, this implementation checks for vars- first, then moves on to the normal css- check.

Checklist:

• Documentation
• Tests
• Code complete

[codecov-io]

Codecov Report

Merging #159 into master will decrease coverage by 0.08%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   90.22%   90.14%   -0.09%     
==========================================
  Files          22       22              
  Lines         706      710       +4     
  Branches      166      168       +2     
==========================================
+ Hits          637      640       +3     
  Misses         56       56              
- Partials       13       14       +1

Impacted Files	Coverage Δ
src/server.js	95% <85.71%> (-5%)	⬇️

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[emmatown]

Could you add some tests? You'll want to add them in the test/extract folder.

[kylpo]

Happy to! Thanks for pointing me to the correct file.

I'm on vacation until Monday. When I get back, I'll work on it.

[kylpo]

Hey, sorry. I haven't forgotten this. Just busy.

[tkh44]

We aren't using vars in v7 so further work on this shouldn't be necessary.