Feature/generic refactor

[BirjuVachhani]

Summary

This lays out ground work for supporting custom ui libraries (e.g. Fluent UI).

• CupertinoAdaptiveThemeManager is now deprecated and replaced with AdaptiveThemeManager<CupertinoThemeData> in favor of supporting theming for other UI frameworks. (e.g. Fluent UI). This will be removed in v4.0.0.
• AdaptiveThemeManager is now generic typed where the generic type represents the type of the theme data object. Replace AdaptiveThemeManager with AdaptiveThemeManager<ThemeData>
• AdaptiveThemeManager is now a mixin instead of an abstract class to reduce code duplication.

[codecov-commenter]

Codecov Report

Merging #40 (0aba9e3) into main (60b8595) will decrease coverage by 1.53%. The diff coverage is 97.50%.

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   98.90%   97.36%   -1.54%     
==========================================
  Files           4        5       +1     
  Lines         183      152      -31     
==========================================
- Hits          181      148      -33     
- Misses          2        4       +2     

Impacted Files	Coverage Δ
lib/src/cupertino_adaptive_theme.dart	95.23% <90.00%> (-4.77%)	:arrow_down:
lib/src/adaptive_theme.dart	100.00% <100.00%> (ø)
lib/src/adaptive_theme_manager.dart	100.00% <100.00%> (ø)
lib/src/adaptive_theme_preferences.dart	96.00% <100.00%> (+0.34%)	:arrow_up:

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