fix: improve parsing of --env flag

[snitin315]

What kind of change does this PR introduce?
bugfix

Did you add tests for your changes?
Yes

If relevant, did you update the documentation?
will add an example in webpack.js.org after merge.

Summary

Fixes #2642

Does this PR introduce a breaking change?
Maybe

Other information
No

[codecov]

Codecov Report

Merging #2643 (b7447c9) into master (d2ab57d) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2643      +/-   ##
==========================================
+ Coverage   95.19%   95.21%   +0.02%     
==========================================
  Files          30       30              
  Lines        1498     1505       +7     
  Branches      429      430       +1     
==========================================
+ Hits         1426     1433       +7     
  Misses         72       72              

Impacted Files	Coverage Δ
packages/webpack-cli/lib/webpack-cli.js	96.17% <100.00%> (+0.03%)	⬆️

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[rishabh3112]

lgtm

[rishabh3112]

There is logical error, for any value containing = this will match, better to look for last char in the string and if it's = then set value as ""

Yup, good catch. One more thing we can do is to introduce some special character for empty string like we do in parser grammars. ` could be tried for this.

[alexander-akait]

Something weird with coverage here

[webpack-bot]

@alexander-akait Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@anshumanv Please review the new changes.

[alexander-akait]

Added more tests + fix edge cases with --verison= (empty string)

[alexander-akait]

After CI will be green I want to merge it, because we need finish #2632 and when finish testing on webpack-dev-server@4, after this we need do release (and webpack-dev-server release too)