fix: add "frozen" class to all slick-pane for easier styling

[ghiscoding]

• prior to this PR, we needed to add frozen broder style on last header/columns, this was not only hard to style, it also wasn't styled correctly in the UI. by adding the "frozen" class directly on every left pane (when frozen column is enabled) makes it much easier to style the UI correctly
• NOTE: since we can now style the frozen pane with 1 line, I deleted a bunch of SASS/CSS variables that are no longer necessary
  • $slick-frozen-preheader-row-border-right
  • $slick-frozen-header-row-border-right
  • $slick-frozen-filter-row-border-right

before

after

notice the blue line representing pinning is now properly displayed across the headers as well

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.8%. Comparing base (8ccd0fa) to head (21d9b1f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1752     +/-   ##
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+ Coverage    99.8%   99.8%   +0.1%     
========================================
  Files         187     187             
  Lines       31121   31129      +8     
  Branches     9780    9783      +3     
========================================
+ Hits        31032   31040      +8     
  Misses         89      89             

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