fix(eslint-plugin): [method-signature-style] fix overloaded methods to an intersection type

[sosukesuzuki]

Fixes #1911

[typescript-eslint]

Thanks for the PR, @sosukesuzuki!

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 #1966 into master will increase coverage by 0.00%.
The diff coverage is 86.95%.

@@           Coverage Diff           @@
##           master    #1966   +/-   ##
=======================================
  Coverage   93.88%   93.89%           
=======================================
  Files         170      170           
  Lines        7737     7765   +28     
  Branches     2227     2236    +9     
=======================================
+ Hits         7264     7291   +27     
+ Misses        218      217    -1     
- Partials      255      257    +2     

Flag	Coverage Δ
#unittest	93.89% <86.95%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
.../eslint-plugin/src/rules/method-signature-style.ts	92.85% <86.95%> (+1.36%)	⬆️
packages/eslint-plugin/src/rules/unbound-method.ts	95.23% <0.00%> (ø)
...kages/eslint-plugin/src/rules/no-base-to-string.ts	98.00% <0.00%> (+0.04%)	⬆️
packages/eslint-plugin/src/rules/return-await.ts	95.94% <0.00%> (+0.23%)	⬆️

[bradzacher]

LGTM - thanks for this.

One change that I just made so I can merge this right now: documented it in the readme.