feat: sharabale configs

[snitin315]

What kind of change does this PR introduce?
FEATURE

Did you add tests for your changes?
yes, more tests WIP
If relevant, did you update the documentation?
WIP
Summary
Fixes #2748

Does this PR introduce a breaking change?
No

Other information

WIP in covering more edge cases with multiple nested configs and multi compiler mode.

[codecov]

Codecov Report

Merging #2868 (8e352fa) into master (ce2c68d) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2868      +/-   ##
==========================================
+ Coverage   94.95%   94.99%   +0.04%     
==========================================
  Files          31       31              
  Lines        1704     1719      +15     
  Branches      484      488       +4     
==========================================
+ Hits         1618     1633      +15     
  Misses         86       86              

Impacted Files	Coverage Δ
packages/webpack-cli/lib/webpack-cli.js	96.81% <100.00%> (+0.04%)	⬆️

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[alexander-akait]

Let's add tests with --merge too

[snitin315]

I am also implementing --extends flag with this.

[alexander-akait]

Yep, it is great idea

[alexander-akait]

We do not use webpack-merge logic here? it is working good

[snitin315]

We will also need to load the extended configurations, here we have only paths for extended configurations and not their options.

Or am I missing something here?

[alexander-akait]

Ideally --extends means --config=webpack.config.js --config=webpack.other.config.js --merge, so I think we already have logic for this, for the next major release we will change logic for multiple --config https://github.com/webpack/webpack-cli/blob/master/packages/webpack-cli/lib/webpack-cli.js#L1619, I think we will throw an error on two and more config options without --merge, but we can reuse this logic for --extends, --extends like syntax sugar, also do not forget --extends in CLI should override extends on webpack.config.js (I think eslint works the same, but need check)

[snitin315]

I will refactor 👍🏻 .

[alexander-akait]

Fixed #3738