Allow the cli to use a string for --maxWorkers

[philiiiiiipp]

Summary

Fixes #8559 and allows --maxWorkers 20% to be used again.

Test plan

I did add tests and added an example to the cli output, however testing the args in specific is one level higher and would require more thought.

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[codecov-io]

Codecov Report

Merging #8565 into master will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #8565      +/-   ##
==========================================
- Coverage    63.2%   63.18%   -0.03%     
==========================================
  Files         271      271              
  Lines       11284    11296      +12     
  Branches     2749     2754       +5     
==========================================
+ Hits         7132     7137       +5     
- Misses       3538     3544       +6     
- Partials      614      615       +1

Impacted Files	Coverage Δ
packages/jest-config/src/Defaults.ts	100% <ø> (ø)	⬆️
packages/jest-config/src/Descriptions.ts	100% <ø> (ø)	⬆️
packages/jest-config/src/ValidConfig.ts	100% <ø> (ø)	⬆️
packages/jest-config/src/normalize.ts	78.2% <100%> (ø)	⬆️
packages/jest-config/src/getMaxWorkers.ts	100% <100%> (ø)	⬆️
packages/jest-validate/src/validate.ts	78.04% <22.22%> (-15.71%)	⬇️

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[jeysal]

Cool, thanks! Kind of uncomfortable not having an integration test for this but at least yargs is pretty declarative so it might be hard to break it in this place again

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