fix(eslint-plugin): [no-base-to-string] support boolean in unions

[auhsoja]

Fixes #1974.

Added an extra check to the no-base-to-string.ts rule to handle the case when a union includes special-case types (specifically, booleans since those tend to be the most common offenders). Whenever we know we have a union, instead of just checking if there is some useful subtype, we also check if all subtypes are useful. If all subtypes are useful, there should be no warning.

Note: This should almost certainly be removed once the boolean toString no longer requires special consideration.

[typescript-eslint]

Thanks for the PR, @auhsoja!

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 #1979 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1979   +/-   ##
=======================================
  Coverage   93.91%   93.92%           
=======================================
  Files         171      171           
  Lines        7789     7798    +9     
  Branches     2240     2243    +3     
=======================================
+ Hits         7315     7324    +9     
  Misses        217      217           
  Partials      257      257           

Flag	Coverage Δ
#unittest	93.92% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
...kages/eslint-plugin/src/rules/no-base-to-string.ts	98.30% <100.00%> (+0.30%)	⬆️

[bradzacher]

LGTM! thanks for this