fix(eslint-plugin): [no-unused-expressions] ignore directives

[a-tarasyuk]

Fixes #1283

[typescript-eslint]

Thanks for the PR, @a-tarasyuk!

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 #1285 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1285      +/-   ##
==========================================
+ Coverage   94.06%   94.06%   +<.01%     
==========================================
  Files         131      131              
  Lines        5745     5746       +1     
  Branches     1617     1618       +1     
==========================================
+ Hits         5404     5405       +1     
  Misses        183      183              
  Partials      158      158

Impacted Files	Coverage Δ
...s/eslint-plugin/src/rules/no-unused-expressions.ts	100% <100%> (ø)	⬆️

[bradzacher]

LGTM - thanks for this!