[New] jsx-pascal-case: add allowLeadingUnderscore option

[mwaddell]

Fixes #2603
Replaces #3039

• Adds a new optional allowLeadingUnderscore property to the existing jsx-pascal-case rule.
• Updates documentation to clearly explain when not to use it
• Updates unit tests to ensure coverage

[ljharb]

I’ll keep both PRs in sync and land them together :-) thanks!

[codecov-commenter]

Codecov Report

Merging #3041 (064afc2) into master (0894132) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3041      +/-   ##
==========================================
+ Coverage   97.38%   97.40%   +0.01%     
==========================================
  Files         111      111              
  Lines        7426     7428       +2     
  Branches     2713     2715       +2     
==========================================
+ Hits         7232     7235       +3     
+ Misses        194      193       -1     

Impacted Files	Coverage Δ
lib/rules/jsx-pascal-case.js	100.00% <100.00%> (+1.51%)	⬆️

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