feat(styling): add frozen on all possible elements with SASS variables

[ghiscoding]

• add frozen to both the pre-header & header columns
• add SASS variables to customize border on these 3 properties (pre-header, header, header-row aka filter row)

[codecov]

Codecov Report

Merging #599 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #599   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          149       149           
  Lines         9946      9949    +3     
  Branches      3516      3519    +3     
=========================================
+ Hits          9946      9949    +3     

Impacted Files	Coverage Δ
...r-slickgrid/services/groupingAndColspan.service.ts	100.00% <100.00%> (ø)

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