[Backport 2.x] Onboard Stylelint (#229)

[ps48]

Description

Adds new yarn lint:style script that runs Stylelint and runs it in the build and test workflow. This should help make sure new Sass isn't written with anti-patterns.

Issues Resolved

#229

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[mengweieric]

~~seeing ```Cannot find module '@babel/plugin-proposal-class-properties'``

[ps48]

Tested locally with cherry-picking this commit:

➜  gantt-chart git:(2.x) ✗ yarn test
yarn run v1.22.10
$ ../../node_modules/.bin/jest --config ./test/jest.config.js
 PASS  public/__tests__/gantt_vis_type.test.ts (9.53 s)
 PASS  public/components/__tests__/options_editor.test.tsx (9.538 s)
 PASS  public/components/__tests__/gantt_chart.test.tsx (9.672 s)
 PASS  public/components/__tests__/gantt_chart_editor.test.tsx (10.333 s)

Test Suites: 4 passed, 4 total
Tests:       19 passed, 19 total
Snapshots:   7 passed, 7 total
Time:        11.924 s
Ran all test suites.
✨  Done in 13.41s.