fix(type-utils): isTypeReadonly now handles conditional types

[RebeccaStevens]

PR Checklist

• Addresses an existing issue: fixes [isTypeReadonly] Conditional types are always marked as readonly #4420
• That issue was marked as accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

isTypeReadonly now checks conditional types properly.

Note: This PR builds off #4416

[nx-cloud]

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✅ Successfully ran 48 targets

• Node 16.x - nx test @typescript-eslint/eslint-plugin-tslint
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[typescript-eslint]

Thanks for the PR, @RebeccaStevens!

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 #4421 (2b275e8) into main (ef3147c) will decrease coverage by 1.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4421      +/-   ##
==========================================
- Coverage   94.23%   92.68%   -1.55%     
==========================================
  Files         169      161       -8     
  Lines        9448     8151    -1297     
  Branches     2948     2616     -332     
==========================================
- Hits         8903     7555    -1348     
- Misses        321      396      +75     
+ Partials      224      200      -24     

Flag	Coverage Δ
unittest	92.68% <100.00%> (-1.55%)	⬇️

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

Impacted Files	Coverage Δ
packages/type-utils/src/isTypeReadonly.ts	79.77% <100.00%> (ø)
packages/eslint-plugin/src/rules/no-extra-semi.ts	54.54% <0.00%> (-12.13%)	⬇️
...lugin-internal/src/rules/plugin-test-formatting.ts
packages/typescript-estree/src/version-check.ts
...ckages/typescript-estree/src/jsx/xhtml-entities.ts
packages/typescript-estree/src/convert.ts
packages/typescript-estree/src/convert-comments.ts
...-internal/src/rules/no-typescript-estree-import.ts
...ript-estree/src/create-program/createSourceFile.ts
...-estree/src/create-program/createProjectProgram.ts
... and 28 more

[JoshuaKGoldberg]

This is great, thanks so much for this & the other issues+PRs @RebeccaStevens! Very much appreciated! 🚀

[bradzacher]

I merged your test PR first - just needs the conflict resolved and we can merge this!

[bradzacher]

man I hate how git handles these types of conflicts.
it's so truly horrible at them.