bug(autocomplete): fix a11y violations

[oykuyilmaz]

Accessibility violations for autocompletion popover are fixed.

• role=listbox is moved to $textLayer of ace_autocomplete editor to make its direct children have role=option, otherwise there is aria-required-children violation (https://dequeuniversity.com/rules/axe/4.5/aria-required-children?application=axeAPI)
• aria-hidden set for textarea of autocomplete popover (there is textarea there since it is an editor instance).
• aria-posinset now starts from 1.

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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (67d318e) 87.24% compared to head (fcab9d7) 87.24%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5241   +/-   ##
=======================================
  Coverage   87.24%   87.24%           
=======================================
  Files         565      565           
  Lines       45283    45284    +1     
  Branches     6928     6928           
=======================================
+ Hits        39505    39506    +1     
  Misses       5778     5778           

Flag	Coverage Δ
unittests	87.24% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/autocomplete/popup.js	85.38% <100.00%> (+0.05%)	⬆️
src/autocomplete_test.js	97.68% <100.00%> (ø)

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