fix(addon): providing columnIndexPosition should always work

[ghiscoding]

• providing columnIndexPosition should work regardless of when the extension (plugin) is created, basically the previous code was assuming that you will always want the checkbox before the row move plugin but that is not always true and the other way around wasn't possible while this PR fixes it

[codecov]

Codecov Report

Merging #794 (4ff9935) into master (5660240) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #794   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          162       162           
  Lines        11325     11340   +15     
  Branches      3983      3884   -99     
=========================================
+ Hits         11325     11340   +15     

Impacted Files	Coverage Δ
...es/angular-slickgrid/services/extension.service.ts	100.00% <100.00%> (ø)
...es/angular-slickgrid/services/gridState.service.ts	100.00% <0.00%> (ø)

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