feat: signal gutterkeydown in keyboard accessibility mode

[akoreman]

Issue #5200

Description of changes: For some applications, users want to react to keys being pressed in the gutter keyboard mode.

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[codecov]

Codecov Report

Patch coverage: 96.29% and project coverage change: +0.01 🎉

Comparison is base (84e653e) 87.17% compared to head (61ce4a8) 87.19%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5202      +/-   ##
==========================================
+ Coverage   87.17%   87.19%   +0.01%     
==========================================
  Files         565      565              
  Lines       45075    45121      +46     
  Branches     6910     6910              
==========================================
+ Hits        39295    39342      +47     
+ Misses       5780     5779       -1     

Flag	Coverage Δ
unittests	87.19% <96.29%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/mouse/default_gutter_handler.js	72.78% <80.00%> (+1.06%)	⬆️
src/keyboard/gutter_handler.js	89.89% <95.23%> (+0.61%)	⬆️
src/keyboard/gutter_handler_test.js	99.46% <100.00%> (+0.09%)	⬆️

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