JIT: Replace @tailwind screens with @tailwind variants

[adamwathan]

This PR adds a @tailwind variants directive as a replacement for the existing @tailwind screens directive.

The @tailwind screens directive has historically been used to control where responsive variants are inserted into the stylesheet. By default we append to the end, but there are some situations where you want to control it, so you have always been able to optionally stick @tailwind screens in your stylesheet and we use that as the insertion point if it exists:

@tailwind base;
@tailwind components;
@tailwind utilities;
@tailwind screens;

/* This will come _after_ the responsive variants now */
.foo {
  color: black;
}

In the JIT engine, there is no real difference between responsive variants or hover/focus/etc. variants — everything is simply a "variant" and all variants are grouped together, unlike in the classic engine where non-responsive variants are grouped per utility.

With this change, the name @tailwind screens no longer feels appropriate, so this PR adds @tailwind variants which does the exact same thing but is better named.

This will probably just sit around quietly as a sort of undocumented thing until 3.0 where we can push this as the recommended name, and deprecate the old one explicitly by showing a warning in the console.

[codecov-commenter]

Codecov Report

Merging #4356 (2d0ac89) into master (87df93d) will increase coverage by 0.04%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #4356      +/-   ##
==========================================
+ Coverage   71.80%   71.84%   +0.04%     
==========================================
  Files         220      220              
  Lines        4802     4806       +4     
  Branches      802      805       +3     
==========================================
+ Hits         3448     3453       +5     
+ Misses       1268     1267       -1     
  Partials       86       86              

Impacted Files	Coverage Δ
src/jit/lib/normalizeTailwindDirectives.js	71.42% <42.85%> (ø)
src/jit/index.js	61.90% <100.00%> (ø)
src/jit/lib/expandTailwindAtRules.js	83.58% <100.00%> (+2.23%)	⬆️

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