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

[a-tarasyuk]

Fixes #2108

[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!

---

🙏 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 #2130 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2130   +/-   ##
=======================================
  Coverage   93.42%   93.42%           
=======================================
  Files         171      171           
  Lines        7755     7755           
  Branches     2215     2215           
=======================================
  Hits         7245     7245           
  Misses        244      244           
  Partials      266      266           

Flag	Coverage Δ
#unittest	93.42% <ø> (ø)

Impacted Files	Coverage Δ
...s/eslint-plugin/src/rules/no-unused-expressions.ts	100.00% <ø> (ø)

[bradzacher]

LGTM.
thanks for this!