feat: IrisGridTheme iconSize

[mattrunyon]

This is part of the work needed for #885 that I felt would be better as a separate PR.

Snapshot changes are going to be slightly shifted sort icon and vertical center + slight left shift on tree table arrows.

[codecov]

Codecov Report

Attention: Patch coverage is 10.00000% with 27 lines in your changes missing coverage. Please review.

Project coverage is 46.62%. Comparing base (50f2a6d) to head (7c7840f).

Files	Patch %	Lines
packages/iris-grid/src/IrisGridRenderer.ts	16.66%	10 Missing ⚠️
...ackages/iris-grid/src/IrisGridCellRendererUtils.ts	0.00%	7 Missing ⚠️
packages/grid/src/GridRenderer.ts	0.00%	4 Missing ⚠️
packages/iris-grid/src/IrisGridIcons.ts	0.00%	3 Missing ⚠️
packages/iris-grid/src/IrisGridTextCellRenderer.ts	0.00%	2 Missing ⚠️
.../mousehandlers/IrisGridCellOverflowMouseHandler.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2123      +/-   ##
==========================================
- Coverage   46.62%   46.62%   -0.01%     
==========================================
  Files         685      685              
  Lines       38492    38493       +1     
  Branches     9590     9589       -1     
==========================================
  Hits        17948    17948              
- Misses      20534    20535       +1     
  Partials       10       10              

Flag	Coverage Δ
unit	46.62% <10.00%> (-0.01%)	⬇️

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