feat: Improved preload variable handling

[bmingles]

• Preload now always applies default preload values before applying cached ones. This ensures that cached preload values don't prevent new defaults from being applied if / when we add new variables to the preload list
• Default preload variables can now be passed in to ThemeUtils + ThemeProvider. This will allow DHE to specify additional variables if needed

resolves #1695 and part of #1679

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a3bea73) 46.44% compared to head (bad2545) 46.45%.

❗ Current head bad2545 differs from pull request most recent head ced4130. Consider uploading reports for the commit ced4130 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1723   +/-   ##
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  Coverage   46.44%   46.45%           
=======================================
  Files         611      611           
  Lines       37204    37209    +5     
  Branches     9363     9364    +1     
=======================================
+ Hits        17279    17284    +5     
  Misses      19871    19871           
  Partials       54       54           

Flag	Coverage Δ
unit	46.45% <100.00%> (+<0.01%)	⬆️

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[mofojed]

You can use the EMPTY_FUNCTION constant instead from @deephaven/utils.

[bmingles]

ah, I went ahead and updated