Keyboard multiselect: support horizontal arrows

[mcsf]

Description

Implements #3244

How Has This Been Tested?

Screenshots (jpeg or gifs if applicable):

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows has proper inline documentation.

[codecov]

Codecov Report

Merging #3245 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3245      +/-   ##
==========================================
- Coverage   31.28%   31.25%   -0.04%     
==========================================
  Files         227      227              
  Lines        6411     6418       +7     
  Branches     1139     1143       +4     
==========================================
  Hits         2006     2006              
- Misses       3699     3702       +3     
- Partials      706      710       +4

Impacted Files	Coverage Δ
editor/writing-flow/index.js	0% <0%> (ø)	⬆️
editor/block-mover/index.js	0% <0%> (ø)	⬆️

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[ellatrix]

Beautiful

[mcsf]

😄