Add Style aliases for Spacing and CornerRadius

[Al12rs]

• Use an alias system for Spacing and CornerRadius which will allow easy Theme updates in the future.
• Added Style classes for StackPanel spacing and Border corner radius.
• Updated StyleGuidelines with the new Palettes and UnifiedIcon
• Updated Focus color name

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.11%. Comparing base (174188e) to head (2a318ae).
Report is 1 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1001   +/-   ##
=======================================
  Coverage   58.10%   58.11%           
=======================================
  Files         644      644           
  Lines       21056    21056           
  Branches     1642     1641    -1     
=======================================
+ Hits        12235    12236    +1     
+ Misses       8432     8428    -4     
- Partials      389      392    +3     

Flag	Coverage Δ
Linux	57.41% <100.00%> (-0.04%)	⬇️
Windows	57.38% <100.00%> (+0.03%)	⬆️
clean_environment_tests	58.09% <100.00%> (+<0.01%)	⬆️
macOS	56.80% <100.00%> (ø)
networking_tests	?

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Files	Coverage Δ
...exusFluentDark/Styles/Controls/Border/Toolbar.xaml	97.50% <100.00%> (ø)
...sFluentDark/Styles/Controls/Button/PillStyles.xaml	98.91% <100.00%> (ø)

... and 7 files with indirect coverage changes