feat(build): incremental build

[redallen]

What: Build only packages whose src directories have changed since the last build. Rename react-tokens/build -> react-tokens/src for this reason, leave react-icons alone for now. Add build:pf3 and build:pf4 build targets.

If you're a PF4 developer all you need to do is yarn start:pf4 and it will rebuild PF4 packages that are out of date. No more yarn build needed for development!

Additional issues: Need to verify this works on CI.

[patternfly-build]

PatternFly-React preview: https://1817-pr-patternfly-react-patternfly.surge.sh

[codecov-io]

Codecov Report

Merging #1817 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1817   +/-   ##
=======================================
  Coverage   82.76%   82.76%           
=======================================
  Files         606      606           
  Lines        6688     6688           
  Branches       76       76           
=======================================
  Hits         5535     5535           
  Misses       1123     1123           
  Partials       30       30

Flag	Coverage Δ
#patternfly3	84.87% <ø> (ø)	⬆️
#patternfly4	79.49% <ø> (ø)	⬆️
#patternflymisc	95.68% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
...ges/patternfly-4/react-tokens/src/templates/esm.js	100% <ø> (ø)
...es/patternfly-4/react-tokens/src/generateTokens.js	93.33% <ø> (ø)
...ges/patternfly-4/react-tokens/src/templates/cjs.js	100% <ø> (ø)
...es/patternfly-4/react-tokens/src/templates/d.ts.js	100% <ø> (ø)

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[tlabaj]

LGTM