chore(core): add UX_BETA feature flag

[kgabryje]

Add UX_BETA feature flag for various UX improvements. Prerequisite for apache/superset#15798

[vercel]

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[codecov]

Codecov Report

Merging #1233 (2bda615) into master (fc0abfa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1233   +/-   ##
=======================================
  Coverage   29.58%   29.58%           
=======================================
  Files         466      466           
  Lines        9565     9565           
  Branches     1572     1572           
=======================================
  Hits         2830     2830           
  Misses       6506     6506           
  Partials      229      229           

Impacted Files	Coverage Δ
...ackages/superset-ui-core/src/utils/featureFlags.ts	100.00% <ø> (ø)

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