fix(eslint-plugin): [camelcase] handle optional member expr

[a-tarasyuk]

Fixes #1196

ESlint#Camelcase

[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!

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[codecov]

Codecov Report

Merging #1204 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1204      +/-   ##
==========================================
+ Coverage    94.1%   94.14%   +0.04%     
==========================================
  Files         124      124              
  Lines        5325     5330       +5     
  Branches     1487     1489       +2     
==========================================
+ Hits         5011     5018       +7     
+ Misses        179      178       -1     
+ Partials      135      134       -1

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/camelcase.ts	90.9% <100%> (+8.76%)	⬆️

[bradzacher]

One nit - otherwise LGTM. Thanks for this

[bradzacher]

Thanks!