feat(core): add custom entry to Select Editor/Filter collections

[ghiscoding]

• add 2 new methods (addCustomFirstEntry, addCustomLastEntry) to optionally add entry on both end of the collection

[codecov]

Codecov Report

Merging #592 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #592    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          149       149            
  Lines         9917      9937    +20     
  Branches      3499      3365   -134     
==========================================
+ Hits          9917      9937    +20     

Impacted Files	Coverage Δ
.../modules/angular-slickgrid/editors/selectEditor.ts	100.00% <100.00%> (ø)
.../modules/angular-slickgrid/filters/selectFilter.ts	100.00% <100.00%> (ø)

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