fix(eslint-plugin): [strict-boolean-expressions] account for truthy literals

[JoshuaKGoldberg]

Fixes #3115.

I hadn't seen this version of the code with phrases like is('nullish', 'string'). Very clever & readable! The code as-is does a lot of loop iterations, but I guess that's fine because type unions are typically very small?

[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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[JoshuaKGoldberg]

Well this is an error I haven't seen in a while 😄 :

[1/4] Resolving packages...
[2/4] Fetching packages...
error An unexpected error occurred: "https://registry.yarnpkg.com/jest-haste-map/-/jest-haste-map-26.6.2.tgz: Request failed \"500 Internal Server Error\"".

[codecov]

Codecov Report

Merging #3236 (42fae56) into master (5414bf2) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3236      +/-   ##
==========================================
- Coverage   92.93%   92.82%   -0.11%     
==========================================
  Files         316      318       +2     
  Lines       10851    10983     +132     
  Branches     3067     3104      +37     
==========================================
+ Hits        10084    10195     +111     
- Misses        342      349       +7     
- Partials      425      439      +14     

Flag	Coverage Δ
unittest	92.82% <100.00%> (-0.11%)	⬇️

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

Impacted Files	Coverage Δ
...int-plugin/src/rules/strict-boolean-expressions.ts	98.78% <100.00%> (+0.07%)	⬆️
packages/eslint-plugin/src/rules/no-unsafe-call.ts	95.23% <0.00%> (-4.77%)	⬇️
...-plugin/src/rules/no-unnecessary-type-assertion.ts	92.30% <0.00%> (-2.36%)	⬇️
...eslint-plugin/src/rules/no-unsafe-member-access.ts	97.82% <0.00%> (-2.18%)	⬇️
...ckages/eslint-plugin/src/rules/no-unsafe-return.ts	98.24% <0.00%> (-1.76%)	⬇️
...es/eslint-plugin/src/rules/no-unsafe-assignment.ts	92.96% <0.00%> (-0.26%)	⬇️
packages/eslint-plugin/src/rules/no-type-alias.ts	100.00% <0.00%> (ø)
...ages/eslint-plugin/src/rules/no-unsafe-argument.ts	81.15% <0.00%> (ø)
...ckages/eslint-plugin/src/util/getThisExpression.ts	84.61% <0.00%> (ø)

[bradzacher]

nice! almost there - one comment

[bradzacher]

sweet - thanks for this!