fix(icon-handling) - Disposing icon after ImageSource is created from its handle

[lilla28]

Changes:

• Added IconUtilites and NativeMethods static classes to convert and dispose Icon appropriately.
• Whitespace changes, added headers
• Moved MorganStanley.ComposeUI.Testing package under shared/tests/TestUtils

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 74.10%. Comparing base (73c48c4) to head (8dd46ec).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #617      +/-   ##
==========================================
+ Coverage   74.05%   74.10%   +0.04%     
==========================================
  Files         241      242       +1     
  Lines        7482     7495      +13     
  Branches      782      890     +108     
==========================================
+ Hits         5541     5554      +13     
+ Misses       1745     1741       -4     
- Partials      196      200       +4     

Files	Coverage Δ
.../MorganStanley.ComposeUI.Testing/TaskExtensions.cs	88.88% <ø> (ø)
src/shell/dotnet/Shell/Utilities/IconUtilities.cs	100.00% <100.00%> (ø)
src/shell/dotnet/Shell/MainWindow.xaml	0.00% <0.00%> (ø)
src/shell/dotnet/Shell/Utilities/DiagnosticInfo.cs	0.00% <0.00%> (ø)
src/shell/dotnet/Shell/App.xaml.cs	0.00% <0.00%> (ø)
src/shell/dotnet/Shell/MainWindow.xaml.cs	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes