feat(core): copy feature flags from main repo

[villebro]

🏆 Enhancements
This copies over the feature flag functionality from apache/superset to make it easier to conditionally enable features in superset-ui.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/superset/superset-ui/AXRJA28LB7nV5sTgEER4LM9wMebo
✅ Preview: https://superset-ui-git-fork-preset-io-villebro-featureflag-superset.vercel.app

[codecov]

Codecov Report

Merging #982 (3f1d0ca) into master (24813c0) will increase coverage by 0.00%.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #982   +/-   ##
=======================================
  Coverage   28.10%   28.10%           
=======================================
  Files         411      412    +1     
  Lines        8405     8411    +6     
  Branches     1190     1193    +3     
=======================================
+ Hits         2362     2364    +2     
- Misses       5890     5894    +4     
  Partials      153      153           

Impacted Files	Coverage Δ
...et-ui-chart-controls/src/shared-controls/index.tsx	42.55% <0.00%> (ø)
...ackages/superset-ui-core/src/utils/featureFlags.ts	100.00% <100.00%> (ø)
...chart-controls/src/shared-controls/dndControls.tsx	35.29% <0.00%> (-6.38%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 24813c0...3f1d0ca. Read the comment docs.

[ktmud]

Nit: flags in tests should probably be test specific because other regular features flags may be eventually cleaned up.

[villebro]

That's a good point, I will address this next time I'm in here