Allow react defaultVersion to be configurable

[onlywei]

I have some projects where I want to detect the react version most of the time, but sometimes I expect that to fail. In those cases, I don't want to default to "latest". I instead want to choose the default fallback version of React.

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.76%. Comparing base (cef8123) to head (79a6496).
Report is 2 commits behind head on master.

Files	Patch %	Lines
lib/util/version.js	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3771      +/-   ##
==========================================
+ Coverage   97.62%   97.76%   +0.13%     
==========================================
  Files         134      134              
  Lines        9617     9644      +27     
  Branches     3488     3497       +9     
==========================================
+ Hits         9389     9428      +39     
+ Misses        228      216      -12     

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[ljharb]

This seems pretty solid, thanks. I'm going to hold off on merging semver-minor PRs for a bit, but I'll make sure to get to it before i make the next release.