Performance: Simplify return value of multi-selection selected selector

[aduth]

Related: #3171, #3170, #3172

This pull request seeks to optimize the rendering of individual blocks when selection occurs. Currently, any time a selection is made, including for a single block, the isSelected prop changes for every block such that it incurs a render on all blocks. Since this change is often from two equivalent states for most blocks (null to false, and vice-versa), the changes here seek to simplify the return value of the selector for determining whether a single block is selected to simply return false when a multi-selection exists (instead of null). This prevents renders except in the blocks included in the multi-selection.

Implementation notes:

The documented behavior of the isBlockSelected was opposite that of its implementation. Both documentation and tests have been updated to reflect what appears to be the intended behavior, but verification would be appreciated.

Testing instructions:

Verify there are no regressions in the behavior of block multi-selection.

[codecov]

Codecov Report

Merging #3173 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3173   +/-   ##
=======================================
  Coverage   31.61%   31.61%           
=======================================
  Files         217      217           
  Lines        6278     6278           
  Branches     1116     1116           
=======================================
  Hits         1985     1985           
  Misses       3609     3609           
  Partials      684      684

Impacted Files	Coverage Δ
editor/selectors.js	95.65% <100%> (ø)	⬆️

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