feat(eslint-plugin): add extension [no-dupe-class-members]

[bradzacher]

Fixes #291

[typescript-eslint]

Thanks for the PR, @bradzacher!

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 #1492 into master will decrease coverage by 0.08%.
The diff coverage is 92.51%.

@@            Coverage Diff            @@
##           master   #1492      +/-   ##
=========================================
- Coverage   95.59%   95.5%   -0.09%     
=========================================
  Files         148     141       -7     
  Lines        6628    6409     -219     
  Branches     1893    1832      -61     
=========================================
- Hits         6336    6121     -215     
+ Misses        111     105       -6     
- Partials      181     183       +2

Impacted Files	Coverage Δ
...nt-plugin/src/rules/indent-new-do-not-use/index.ts	98.4% <ø> (ø)	⬆️
...s/eslint-plugin/src/rules/interface-name-prefix.ts	100% <ø> (+16.66%)	⬆️
...nt-plugin/src/rules/no-unused-vars-experimental.ts	91.39% <ø> (ø)	⬆️
...ges/eslint-plugin/src/rules/no-inferrable-types.ts	93.1% <ø> (ø)	⬆️
...ges/experimental-utils/src/ts-eslint/SourceCode.ts	100% <ø> (ø)	⬆️
packages/eslint-plugin/src/rules/no-type-alias.ts	100% <ø> (ø)	⬆️
...nt-plugin/src/rules/no-extra-non-null-assertion.ts	100% <100%> (ø)	⬆️
.../eslint-plugin/src/rules/triple-slash-reference.ts	96.66% <100%> (+0.11%)	⬆️
packages/typescript-estree/src/convert-comments.ts	100% <100%> (ø)	⬆️
...-plugin/src/rules/explicit-function-return-type.ts	100% <100%> (ø)	⬆️
... and 32 more