fix(eslint-plugin): [no-type-alias] handle readonly types in aliases

[nicolashenry]

Fixes #1981

[typescript-eslint]

Thanks for the PR, @nicolashenry!

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!

---

🙏 Please, if you or your company is finding typescript-eslint valuable, help us sustain the project by sponsoring it transparently on https://opencollective.com/typescript-eslint. As a thank you, your profile/company logo will be added to our main README which receives thousands of unique visitors per day.

[codecov]

Codecov Report

Merging #1990 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1990   +/-   ##
=======================================
  Coverage   93.91%   93.91%           
=======================================
  Files         171      171           
  Lines        7789     7789           
  Branches     2240     2240           
=======================================
  Hits         7315     7315           
  Misses        217      217           
  Partials      257      257           

Flag	Coverage Δ
#unittest	93.91% <ø> (ø)

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/no-type-alias.ts	100.00% <ø> (ø)

[bradzacher]

lgtm! thanks