Autocomplete accessibility features

[ghost]

Issue #, if available:

Description of changes:
Adds necessary roles and aria-labels for the autocomplete popup.

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[codecov]

Codecov Report

Base: 86.11% // Head: 86.11% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (9dac4d5) compared to base (5016e90).
Patch coverage: 93.93% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5008   +/-   ##
=======================================
  Coverage   86.11%   86.11%           
=======================================
  Files         546      546           
  Lines       41600    41627   +27     
  Branches     6553     6555    +2     
=======================================
+ Hits        35822    35847   +25     
- Misses       5778     5780    +2     

Flag	Coverage Δ
unittests	86.11% <93.93%> (+<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	78.47% <86.66%> (+0.38%)	⬆️
src/autocomplete.js	65.25% <100.00%> (+0.18%)	⬆️
src/autocomplete_test.js	97.50% <100.00%> (ø)
src/keyboard/textinput.js	75.94% <100.00%> (+0.53%)	⬆️
src/layer/text.js	91.52% <100.00%> (+0.01%)	⬆️

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[skazantsev]

In the current implementation we set an id of the selected element in the autocomplete and remove the id once the element is not selected/focussed. Why we need to do all of this? Can't we generate id once e.g. ace_line_{index} and set aria-activedescendant when an item is selected? Ofc, we need to make sure that ids do not clash.