fix: refactored highlightedIndex on Arrow keydown

[silviuaavram]

Initiate highlightedIndex on ArrowUp and ArrowDown open.
Refactored how the index is calculated.
If user passes anything related to highlightedIndex, then we don't change its value on open, even on Arrow Up/Down. This includes controlled highlightedIndex, defaultHighlightedIndex or highlightFirstItemOnOpen.
Fixes #1176

Avoided major refactoring in this area as I want to use hooks in the future rather than have this logic here anymore.

[codecov]

Codecov Report

Merging #1570 into master will decrease coverage by 0.02%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #1570      +/-   ##
==========================================
- Coverage   71.66%   71.63%   -0.03%     
==========================================
  Files         848      848              
  Lines        6910     6910              
  Branches     1963     1981      +18     
==========================================
- Hits         4952     4950       -2     
- Misses       1952     1954       +2     
  Partials        6        6

Impacted Files	Coverage Δ
...ackages/react/src/components/Dropdown/Dropdown.tsx	88.1% <90%> (-0.51%)	⬇️

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