Fix portal bug in React v17

[WesCossick]

Closes #216. Fixed by wrapping document.addEventListener() in a setTimeout().

While I was at it, I also prevented the useEffect() Hook from adding event listeners whenever the menu is closed.

[codecov]

Codecov Report

Merging #217 (b272c46) into master (69a25ca) will increase coverage by 1.41%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   80.72%   82.14%   +1.41%     
==========================================
  Files           1        1              
  Lines          83       84       +1     
  Branches       22       22              
==========================================
+ Hits           67       69       +2     
+ Misses         16       15       -1     

Impacted Files	Coverage Δ
src/use-dropdown-menu.ts	82.14% <75.00%> (+1.41%)	⬆️

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