feat: allow string value for '--hot'

[snitin315]

What kind of change does this PR introduce?
feature

Did you add tests for your changes?
yes, serve tests can be added after upgrade to v4.
If relevant, did you update the documentation?
Yes
Summary

Allow string value for --hot, --hot=only is accepted by webpack-dev-server

Does this PR introduce a breaking change?
No

Other information
No

[codecov]

Codecov Report

Merging #2444 (db3c6a6) into master (74ed07b) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2444      +/-   ##
==========================================
+ Coverage   71.86%   71.89%   +0.03%     
==========================================
  Files          46       46              
  Lines        2182     2185       +3     
  Branches      581      582       +1     
==========================================
+ Hits         1568     1571       +3     
  Misses        614      614              

Impacted Files	Coverage Δ
packages/webpack-cli/lib/webpack-cli.js	91.95% <100.00%> (+0.02%)	⬆️

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