Embed JIT engine directly into Tailwind

[adamwathan]

This PR takes the @tailwindcss/jit project and embeds it directly into Tailwind itself behind a new configuration option:

// tailwind.config.js
module.exports = {
  mode: 'jit',
  // ...
}

This feature is still in preview/experimental and is not subject to semver as we continue to work out the kinks.

[codecov-io]

Codecov Report

Merging #3905 (40165b7) into master (60b5d63) will decrease coverage by 6.57%.
The diff coverage is 83.14%.

@@            Coverage Diff             @@
##           master    #3905      +/-   ##
==========================================
- Coverage   93.34%   86.77%   -6.58%     
==========================================
  Files         178      311     +133     
  Lines        1849     4235    +2386     
  Branches      332      788     +456     
==========================================
+ Hits         1726     3675    +1949     
- Misses        105      485     +380     
- Partials       18       75      +57     

Impacted Files	Coverage Δ
jit/corePlugins/borderRadius.js	18.00% <18.00%> (ø)
jit/corePlugins/borderWidth.js	31.03% <31.03%> (ø)
jit/corePlugins/scale.js	42.10% <42.10%> (ø)
jit/lib/utils.js	48.64% <48.64%> (ø)
src/index.js	82.05% <55.55%> (-8.58%)	⬇️
jit/lib/setupContext.js	70.53% <70.53%> (ø)
jit/lib/rewriteTailwindImports.js	71.42% <71.42%> (ø)
jit/index.js	71.87% <71.87%> (ø)
jit/lib/expandTailwindAtRules.js	81.42% <81.42%> (ø)
jit/corePlugins/transitionProperty.js	83.33% <83.33%> (ø)
... and 278 more

---

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[excitedbox]

How would this be implemented into something like a page builder? For example, the builder uses a full tailwind CSS file and on saving it would prune the CSS file in a certain subdirectory / path. Would it be easier to generate a separate config in each subdirectory and run the compiler over that instead?