Core - Simplify custom webpack config

[igor-dv]

Issue: Partially related to #4903

Today we have an option to extend webpack config by providing the function like this:

module.exports = (basicConfig, mode, defaultConfig) => { /*...*/ }

Where defaultConfig is a few rules bigger than a basicConfig. I think it brings more confusion rather a benefit.

What I did

1. Reduced the basicConfig and changed to the signature to:

module.exports = ({ config, mode }) => { /*...*/ }

Why object? Because it's much easier to extend and deprecate in the future.

2. Removed a minor prop (defaultConfigName) that is passed forme the frameworks to core. This prop is kinda not needed.

[codecov]

Codecov Report

Merging #4927 into next will increase coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##             next    #4927      +/-   ##
==========================================
+ Coverage   35.02%   35.04%   +0.02%     
==========================================
  Files         566      566              
  Lines        7004     7000       -4     
  Branches      937      936       -1     
==========================================
  Hits         2453     2453              
+ Misses       4055     4052       -3     
+ Partials      496      495       -1

Impacted Files	Coverage Δ
app/ember/src/server/options.js	0% <ø> (ø)	⬆️
app/react/src/server/options.js	0% <ø> (ø)	⬆️
app/angular/src/server/options.js	0% <ø> (ø)	⬆️
...b/core/src/server/preview/custom-webpack-preset.js	0% <0%> (ø)	⬆️

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[storybook-safe-bot]

	Fails
🚫	PR is marked with "do not merge", "BREAKING CHANGE" labels.

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[ndelangen]

Shall we merge this into the overhaul-ui branch?

[Hypnosphi]

What is the recommended way of getting only the absolutely required part of default config now?