fix(eslint-plugin): [no-floating-promises] handle finally callback

[a-tarasyuk]

Fixes #1603

[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 #1620 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1620      +/-   ##
==========================================
+ Coverage   95.54%   95.55%   +<.01%     
==========================================
  Files         142      142              
  Lines        6583     6585       +2     
  Branches     1883     1884       +1     
==========================================
+ Hits         6290     6292       +2     
  Misses        106      106              
  Partials      187      187

Impacted Files	Coverage Δ
...es/eslint-plugin/src/rules/no-floating-promises.ts	100% <100%> (ø)	⬆️

[bradzacher]

LGTM - thanks for this!