fix: spreading 2 grids options causes duplicate ext resources

[ghiscoding]

• partially rolls back PR fix: registered external resouces should keep singleton ref #1118 because that caused a regression that was identified by a user of Angular-Slickgrid in which defining a global external resource for ExcelExport no longer worked.
• this PR also fixes duplicatation of external resources when having multiple grid on the same page and spreading grid options of another grid (e.g.: { ...this.gridOptions1, sortable: false })

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (9ae758c) 22.74% compared to head (b4fcf69) 22.64%.

Files	Patch %	Lines
...lia-slickgrid/custom-elements/aurelia-slickgrid.ts	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1126      +/-   ##
==========================================
- Coverage   22.74%   22.64%   -0.10%     
==========================================
  Files           8        8              
  Lines         893      897       +4     
  Branches      313      316       +3     
==========================================
  Hits          203      203              
- Misses        690      694       +4     

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