feat(eslint-plugin): [unbound-method] add support for methods with a this: void parameter

[ryota-ka]

Fixes #854

Despite the original issue has the enhancement: plugin rule option label, this looks like just a false positive to me.
Feel free to give feedbacks!

[typescript-eslint]

Thanks for the PR, @ryota-ka!

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 #2796 (c343350) into master (50a46c6) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #2796      +/-   ##
==========================================
- Coverage   92.80%   92.79%   -0.02%     
==========================================
  Files         300      300              
  Lines        9857     9863       +6     
  Branches     2769     2773       +4     
==========================================
+ Hits         9148     9152       +4     
  Misses        332      332              
- Partials      377      379       +2     

Flag	Coverage Δ
unittest	92.79% <75.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/unbound-method.ts	93.10% <75.00%> (-1.96%)	⬇️

[bradzacher]

The reason that I labelled this as an enhancement is because this rule previously does not care about this args.

So adding support for this args is an enhancement to the rule.

[bradzacher]

thanks for adding this!