fix: show a warning if serve and --watch are used together

[snitin315]

What kind of change does this PR introduce?
fix

Did you add tests for your changes?
yes
If relevant, did you update the documentation?
no
Summary
fixes #2370
show a warning if serve and --watch are used together

Does this PR introduce a breaking change?
No

Other information
I will work on removing --watch & --no-watch options from serve help output in next PR.

[codecov]

Codecov Report

Merging #2375 (81288b1) into master (f9ce1d3) will increase coverage by 0.74%.
The diff coverage is 83.70%.

@@            Coverage Diff             @@
##           master    #2375      +/-   ##
==========================================
+ Coverage   70.52%   71.26%   +0.74%     
==========================================
  Files          47       46       -1     
  Lines        2158     2158              
  Branches      563      570       +7     
==========================================
+ Hits         1522     1538      +16     
+ Misses        636      620      -16     

Impacted Files	Coverage Δ
packages/webpack-cli/bin/cli.js	44.44% <33.33%> (-10.56%)	⬇️
...kages/webpack-cli/lib/utils/get-package-manager.js	61.53% <50.00%> (ø)
packages/webpack-cli/lib/utils/index.js	82.60% <82.60%> (ø)
packages/webpack-cli/lib/webpack-cli.js	91.08% <84.84%> (-0.19%)	⬇️
packages/serve/src/index.ts	84.21% <100.00%> (ø)
packages/webpack-cli/lib/bootstrap.js	100.00% <100.00%> (ø)
packages/webpack-cli/lib/index.js	100.00% <100.00%> (ø)
packages/webpack-cli/lib/plugins/CLIPlugin.js	100.00% <100.00%> (ø)
packages/webpack-cli/lib/utils/logger.js	100.00% <100.00%> (ø)
packages/webpack-cli/lib/utils/package-exists.js	100.00% <100.00%> (ø)
... and 1 more

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[alexander-akait]

It should be inside serve command, because only servecommand know what it is unnecessary

[alexander-akait]

Ideally we should:

• remove --watch and --no-watch for serve and watch, so developers will not use it done
• show warning when configuration has watch: true and set it to false to avoid Deprecation when you use serve or watch done

I think it is the best DX