fix(webpack-dev-server): parse env args

[jdbranham]

SUMMARY

The --superset arg is rejected by webpack because it's not well known.

This PR updates the documentation and changes the flag to use the --env arg as a way to override the backend superset environment when running the webpack-dev-server.

TESTING INSTRUCTIONS

From the superset-frontend directory, run the command to use a remote superset backend.

npm run dev-server -- --env=--superset=https://my-superset-backend

ADDITIONAL INFORMATION

• Has associated issue: Fixes webpack-cli throws 'Unknown option' error when using --superset arg #19743
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[vivostar]

+1, the patch work well on my local dev environment, thanks @jdbranham

[TacoBel42]

that solved connection problem, thx!

[rusackas]

Sorry this never got a review somehow. Adding a few folks to review/test this. I'm also closing/reopening to kick-start CI, and see if we can get this thing across the finish line.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ae70212) 66.51% compared to head (2c4396c) 68.43%.
Report is 3191 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19744      +/-   ##
==========================================
+ Coverage   66.51%   68.43%   +1.92%     
==========================================
  Files        1687     1840     +153     
  Lines       64623    70219    +5596     
  Branches     6646     8275    +1629     
==========================================
+ Hits        42983    48054    +5071     
- Misses      19940    20114     +174     
- Partials     1700     2051     +351     

Flag	Coverage Δ
javascript	56.91% <ø> (+5.72%)	⬆️

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[rusackas]

LGTM!