feat(webpack-cli): add arg postprocessing

[ylemkimon]

What kind of change does this PR introduce?
Feature

Did you add tests for your changes?
Yes

If relevant, did you update the documentation?
N/A

Summary

• Added arg.processor for post-processing options, e.g., migration, deprecation, renaming, nesting, etc.
  • Examples: https://github.com/webpack/webpack-dev-server/pull/2857/files#diff-6639cb4cd2a1457aacae2c81ab3040c3c3fd959572d96019fffc5294ca34411e
• Removed createConfig in favor of webpack-dev-server/bin/cli-flags

Does this PR introduce a breaking change?
@webpack-cli/server/lib/createConfig is removed.

Other information

[codecov]

Codecov Report

Merging #2133 (e3d1d9f) into master (2cd4f1f) will decrease coverage by 0.05%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2133      +/-   ##
==========================================
- Coverage   68.70%   68.65%   -0.06%     
==========================================
  Files          74       73       -1     
  Lines        2390     2386       -4     
  Branches      512      512              
==========================================
- Hits         1642     1638       -4     
  Misses        748      748              

Impacted Files	Coverage Δ
packages/webpack-cli/lib/utils/arg-parser.js	89.79% <42.85%> (-3.62%)	⬇️
packages/serve/src/startDevServer.ts	87.09% <100.00%> (-0.79%)	⬇️

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