Add a go to first/last menu-item helper method in the Menu class

[Snuffleupagus]

This fixes a bug when using the Home and End keyboard shortcuts to navigate through a Menu instance. These two buttons didn't update the #lastIndex field, which means that e.g. a following ArrowDown or ArrowUp press could make focus "jump" to an unexpected menu-item.

Also, the helper method reduces a little bit of code duplication in the event handlers.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.02%. Comparing base (b4ba666) to head (fc610d3).

Files with missing lines	Patch %	Lines
web/menu.js	66.66%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21690   +/-   ##
=======================================
  Coverage   90.01%   90.02%           
=======================================
  Files         264      264           
  Lines       66893    66894    +1     
=======================================
+ Hits        60214    60219    +5     
+ Misses       6679     6675    -4     

Flag	Coverage Δ
browsertest	66.51% <ø> (-0.03%)	⬇️
integrationtest	69.36% <66.66%> (+0.02%)	⬆️
unittest	57.89% <ø> (+<0.01%)	⬆️
unittestcli	56.62% <ø> (+0.01%)	⬆️

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[calixteman]

LGTM. It'd be nice to have an integration test too, but if you don't have time to do one, please tell me and I'll do it myself in a follow-up.
Thank you.

[Snuffleupagus]

It'd be nice to have an integration test too, but if you don't have time to do one, please tell me and I'll do it myself in a follow-up.

It would be great if you could do that in a follow-up, thank you!