Fix deep element not allowed to be disabled being returned as disabled

[lourenci]

Elements not allowed to be disabled are returning as disabled when inside a disabled fieldset.

<fieldset disabled>
  // <a /> is enabled even when inside a disabled fieldset
  <a />
</fieldset>

• Documentation
• Tests
• Updated Type Definitions
• Ready to be merged
  

[codecov]

Codecov Report

Merging #261 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #261   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines          313       315    +2     
  Branches        73        71    -2     
=========================================
+ Hits           313       315    +2     

Impacted Files	Coverage Δ
src/to-be-disabled.js	100.00% <100.00%> (ø)

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@all-contributors add @lourenci for code, test, bug

[allcontributors]

@gnapse

I've put up a pull request to add @lourenci! 🎉

[gnapse]

🎉 This PR is included in version 5.10.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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