Core: Add Babel mode v7

[shilman]

Issue: #14425 #2758 #12745 #15502 #12292

This PR overhauls Storybook's babel handling in preparation for v7.0.

Before:

• Storybook provides a baseline config
• User config is inconsistently respected (sometimes respected and sometimes not!)
• Addons and frameworks add incremental modifications to config
• Users recommended to override babel in main.js
• NO debugging methodology

After (when features.babelModeV7 flag is activated):

• Storybook provides NO baseline config
• Users can ask Storybook to generate a baseline config file (.babelrc.json)
• User config is respected
• Addons and frameworks add incremental modifications to config
• Users recommended to edit their project babel config, can worst-case override it in main.js webpackFinal
• Debugging methodology in docs

What I did

• Add features.babelModeV7
• Update webpack4/5 builder/manager changes
  • DON'T load storybook babel defaults in the new mode
  • DO configure babel loader to look for the project babelrc/babel config
• Fix web-components babel handling for consistency
• Automatically generate babelrc on empty project sb init
• CLI command for force-generating babelrc
• Open question: do incremental modifications to babel config actually work??
• Migration instructions
• Documentation w/ debug instructions

Potential improvements

• Fix CLI dependency structure
• Automatically set V7 feature flag on empty projects

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

The easiest way to test this is to modify the .babelrc of examples/react-ts, which has babelModeV7 enabled. You can:

• Manually edit the babelrc
• Remove the babelrc and generate one using ../../lib/cli/bin/index.js
• De-activate the babelModeV7 flag
• Activate the babelModeV7 flag in examples/cra-kitchen-sink

You can also do all of this with BABEL_SHOW_CONFIG_FOR enabled to see what's going on under the hood. But you can't actually run the Storybooks this way -- see the documentation.

If your answer is yes to any of these, please make sure to include it in your PR.

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[shilman]

self-merging @tmeasday @ndelangen