InsertionSort: use index to compare instead undefined

[SidKwok]

1. use currentIndex - 1 >= 0 instead array[currentIndex - 1] !== undefined in while-loop, I think it should be more readable.
2. swap value with destructuring assignment

some thoughts:
Since the for-loop in InsertionSort should start from 1(in 0 it will just pass the while-loop), I think for (let i = 1; i < array.length; i += 1) would be a better idea.

[codecov]

Codecov Report

Merging #231 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #231   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         143    143           
  Lines        2554   2552    -2     
  Branches      422    422           
=====================================
- Hits         2554   2552    -2

Impacted Files	Coverage Δ
...algorithms/sorting/insertion-sort/InsertionSort.js	100% <100%> (ø)	⬆️

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