Include only necessary fonts for target environment specified by Browserslist

[ylemkimon]

Fixes #1667.
Closes #1495.

+ Allow WOFF and WOFF2 to be controlled using environment variables like TTF. (USE_TTF) These environment variables will override target environment.

Uses caniuse data to detect support. No additional dependencies are required, as they are already included in other dependencies.

[codecov]

Codecov Report

Merging #1674 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1674   +/-   ##
=======================================
  Coverage   93.67%   93.67%           
=======================================
  Files          80       80           
  Lines        4666     4666           
  Branches      813      813           
=======================================
  Hits         4371     4371           
  Misses        261      261           
  Partials       34       34

Flag	Coverage Δ
#screenshotter	88.62% <ø> (ø)	⬆️
#test	85.04% <ø> (ø)	⬆️

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[kevinbarabash]

I tried BROWSERSLIST="Chrome 68" USE_WOFF2=true yarn build and the fonts worked. The dist/fonts only contained .woff2 files. I checked dist/katex.js and the code had been compiled down to ES5. I tried BROWSERSLIST="Chrome 68" yarn build with the same results.

[ylemkimon]

@kevinbarabash There was a bug in #1595, where Browserslist config is not read even if isESMBuild is false. Also, I've upgraded uglifyjs-webpack-plugin to terser-webpack-plugin, to be able to minify ES6 output.

[codecov-io]

Codecov Report

Merging #1674 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1674   +/-   ##
=======================================
  Coverage   93.66%   93.66%           
=======================================
  Files          80       80           
  Lines        4653     4653           
  Branches      807      807           
=======================================
  Hits         4358     4358           
  Misses        261      261           
  Partials       34       34

Flag	Coverage Δ
#screenshotter	88.85% <ø> (ø)	⬆️
#test	84.99% <ø> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[kevinbarabash]

Sorry for the delay, I finally got around to testing this again. The browserslist setting is working correctly. Thanks for adding this feature. I'm personally looking forward to using this at Khan Academy once we finish our migration to webpack.