fix(eslint-plugin): [prefer-regexp-exec] factor in union types

[JoshuaKGoldberg]

Fixes #3319.

Edit 5/31: the tests were previously asserting on non-string (e.g. number) values getting a rule complaint... but that seems off? TypeScript should let us know anyway that /abc/.match(null) isn't allowed. Updated the logic to not report on them.

Co-authored-by: @IllusionMH

[typescript-eslint]

Thanks for the PR, @JoshuaKGoldberg!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #3434 (f03a4a4) into master (b3ac5f6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3434   +/-   ##
=======================================
  Coverage   92.63%   92.64%           
=======================================
  Files         325      325           
  Lines       11235    11247   +12     
  Branches     3168     3169    +1     
=======================================
+ Hits        10408    10420   +12     
  Misses        367      367           
  Partials      460      460           

Flag	Coverage Δ
unittest	92.64% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ages/eslint-plugin/src/rules/prefer-regexp-exec.ts	100.00% <100.00%> (ø)

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Status	Command
	nx run-many --target=build --all --parallel
	nx run-many --target=typecheck --all --parallel

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[bradzacher]

LGTM - thanks!