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)

• fix(core): don't override alwaysShowVerticalScroll flag

  • the alwaysShowVerticalScroll flag should be dealt directly in the core and it should never show vertical scroll on left frozen container, even when the alwaysShowVerticalScroll is set to True (see core lib PR #537)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0086402). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             master      #138   +/-   ##
==========================================
  Coverage          ?   100.00%           
==========================================
  Files             ?       196           
  Lines             ?     11256           
  Branches          ?      3691           
==========================================
  Hits              ?     11256           
  Misses            ?         0           
  Partials          ?         0           

Impacted Files	Coverage Δ
...ackages/common/src/extensions/gridMenuExtension.ts	100.00% <100.00%> (ø)
...kages/common/src/extensions/headerMenuExtension.ts	100.00% <100.00%> (ø)
.../common/src/services/groupingAndColspan.service.ts	100.00% <100.00%> (ø)

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