CellMeasurer: add registerChild render prop

[Hypnosphi]

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Motivation

This enables a way to use CellMeasurer without findDOMNode, e.g. in React StrictMode. Here's an example usage:

<CellMeasurer {...}>
  {({registerChild}) => <div ref={registerChild} />}
</CellMeasurer>

[codecov-io]

Codecov Report

Merging #1477 into master will decrease coverage by 0.03%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1477      +/-   ##
==========================================
- Coverage    94.7%   94.66%   -0.04%     
==========================================
  Files          47       47              
  Lines        1795     1801       +6     
==========================================
+ Hits         1700     1705       +5     
- Misses         95       96       +1

Impacted Files	Coverage Δ
source/CellMeasurer/CellMeasurer.js	93.02% <85.71%> (-1.58%)	⬇️

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[Hypnosphi]

@wuweiweiwu @bvaughn Can you please review? React Concurrent mode gets closer every day =)

[wuweiweiwu]

Thank you for this!! Will release as a minor

[seanstrom]

@wuweiweiwu when is this change planned to be released?

[fake-car]

please release to appease

[wuweiweiwu]

apologize for the super delayed release

https://github.com/bvaughn/react-virtualized/blob/master/CHANGELOG.md#9220