feat(sort): add valueCouldBeUndefined column flag to help sorting

[ghiscoding]

• this is an opt-in flag because it could bring unwanted behavior and for example it breaks the Row Detail UI when sorting undefined values, so user can opt-in but it's off by default

[codecov]

Codecov Report

Merging #429 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #429    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          141       141            
  Lines         8888      8891     +3     
  Branches      2942      3073   +131     
==========================================
+ Hits          8888      8891     +3     

Impacted Files	Coverage Δ
...modules/angular-slickgrid/sorters/dateUtilities.ts	100.00% <100.00%> (ø)
...modules/angular-slickgrid/sorters/numericSorter.ts	100.00% <100.00%> (ø)
...dules/angular-slickgrid/sorters/sorterUtilities.ts	100.00% <100.00%> (ø)
.../modules/angular-slickgrid/sorters/stringSorter.ts	100.00% <100.00%> (ø)

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