[WIP] Fix entries syntax

[alexander-akait]

What kind of change does this PR introduce?

feature

Did you add tests for your changes?

Yes

If relevant, did you update the documentation?

Yes:

• webpack ./entry.js --mode=development
• webpack ./entry.js ./other.js --mode=development
• webpack build ./entry.js --mode=development
• webpack build ./entry.js ./other.js --mode=development
• webpack watch ./entry.js --mode=development
• webpack serve ./entry.js --mode=development

Summary

fixes #2356

Does this PR introduce a breaking change?

No

Other information

WIP

[codecov]

Codecov Report

Merging #2369 (cfeb158) into master (c5a06f7) will increase coverage by 0.40%.
The diff coverage is 96.65%.

@@            Coverage Diff             @@
##           master    #2369      +/-   ##
==========================================
+ Coverage   70.09%   70.50%   +0.40%     
==========================================
  Files          48       47       -1     
  Lines        2117     2156      +39     
  Branches      548      561      +13     
==========================================
+ Hits         1484     1520      +36     
- Misses        633      636       +3     

Impacted Files	Coverage Δ
packages/webpack-cli/lib/webpack-cli.js	91.25% <96.60%> (+0.32%)	⬆️
packages/serve/src/index.ts	84.21% <100.00%> (+0.57%)	⬆️

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[evenstensberg]

lgtm

[alexander-akait]

Still WIP, entry=./entry.js is not supported here

[alexander-akait]

Impossible to implement:

- `webpack entry=./entry.js --mode=development` (WIP)
- `webpack entry=./entry.js other=./other.js --mode=development` (WIP)

We need new this on webpack side, webpack accept only string or array of strings (put in my todo)