feat(Dropdown): arrow navigation between selected items

[silviuaavram]

Allows the navigation between Dropdown selected items using the Arrow Left and Arrow Right keys. Dropdown must be a multiple selection.

Removed the tabIndex=0 from Label as it was not needed for click. @mnajdova please confirm. As a result, 'X' icons are no longer in the tab order, they can only be clicked in order to remove the item. Focus is returned to the search input at removal.

Selected items are focusable now, but are not part of the normal tab order. Can be focused by click or by left/right arrow keys.

• Left Arrow from search input should focus last selected item.
• Right Arrow from last selected item should focus input.
• Left/Right Arrows should switch focus between the items, depending on the currently focused one.
• Delete/Backspace while focus is on a selected item should remove that item and put focus on the search input.

Added a style similar to Teams to the focused item (background colour change).

May need to rename activeIndex as I used it for consistency with Accordion/Menu but it may be a bit confusing, accepting feedback.

Adding screener test to check highlighted style on both keydown and click.

[codecov]

Codecov Report

Merging #842 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #842   +/-   ##
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  Coverage   92.99%   92.99%           
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  Files          21       21           
  Lines         728      728           
  Branches       73       73           
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  Hits          677      677           
  Misses         51       51

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[bmdalex]

@silviuavram - generally looks good, one thing: as we discussed, we'd like have the key nav extracted in the behaviors, can you create an issue or if it already exists reference it in this PR? Thanks!