fix(eslint-plugin): [prefer-readonly-parameter-types] handle recursive types

[bradzacher]

Fixes #1665

Also did a small refactor to use an enum instead of boolean | null, because it's cleaner.

[typescript-eslint]

Thanks for the PR, @bradzacher!

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 #1672 into master will decrease coverage by <.01%.
The diff coverage is 97.14%.

@@            Coverage Diff             @@
##           master    #1672      +/-   ##
==========================================
- Coverage   95.35%   95.35%   -0.01%     
==========================================
  Files         148      148              
  Lines        6918     6931      +13     
  Branches     1989     1992       +3     
==========================================
+ Hits         6597     6609      +12     
  Misses        112      112              
- Partials      209      210       +1

Impacted Files	Coverage Δ
packages/eslint-plugin/src/util/isTypeReadonly.ts	97.01% <97.14%> (-1.14%)	⬇️