Add --debug-webpack option

[shilman]

Issue: #6081

What I did

--debug-webpack prints out both the final manager and preview webpack configs to help people debug their webpack configuration

NOTE: Though this is technically a feature request, I propose that we patch it into 5.0.x and 4.1.x to help people migrate their webpack configs.

How to test

cd examples/official-storybook
yarn storybook --debug-webpack
yarn build-storybook --debug-webpack

[codecov]

Codecov Report

Merging #6082 into next will decrease coverage by 0.05%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##             next    #6082      +/-   ##
==========================================
- Coverage   37.11%   37.05%   -0.06%     
==========================================
  Files         648      649       +1     
  Lines        9523     9536      +13     
  Branches     1352     1385      +33     
==========================================
  Hits         3534     3534              
- Misses       5410     5419       +9     
- Partials      579      583       +4

Impacted Files	Coverage Δ
lib/core/src/server/cli/dev.js	0% <ø> (ø)	⬆️
lib/core/src/server/cli/prod.js	0% <ø> (ø)	⬆️
lib/core/src/server/build-static.js	0% <0%> (ø)	⬆️
lib/core/src/server/dev-server.js	0% <0%> (ø)	⬆️
lib/core/src/server/logConfig.js	0% <0%> (ø)

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[tmeasday]

LGTM

[wuweiweiwu]

LGTM this is so useful!!