Fix MissingMethodException in UndockedRegFreeWinRT when creating msixbundles with trimming enabled

[Copilot]

Problem

Applications published as msixbundles with trimming enabled (PublishTrimmed=true) crash on startup with the following exception:

System.TypeInitializationException: The type initializer for '<Module>' threw an exception.
 ---> System.MissingMethodException: Method not found: 'Void System.Environment.SetEnvironmentVariable(System.String, System.String)'.
   at Microsoft.Windows.Foundation.UndockedRegFreeWinRTCS.AutoInitialize.AccessWindowsAppSDK()

This happens because the Environment.SetEnvironmentVariable method is trimmed out during the publishing process when PublishTrimmed is set to true.

Solution

This PR implements two key improvements to fix the issue:

1. Added a try-catch block around the Environment.SetEnvironmentVariable call to handle the MissingMethodException gracefully.
2. Added a fallback mechanism using P/Invoke to call the Win32 SetEnvironmentVariableW function directly when the .NET method is not available.

The fallback approach ensures the environment variable gets set even in trimmed environments, allowing the application to start properly. This is important for PublishSingleFile support which relies on the MICROSOFT_WINDOWSAPPRUNTIME_BASE_DIRECTORY environment variable for SxS redirection.

Changes

• Modified UndockedRegFreeWinRT-AutoInitializer.cs to catch the MissingMethodException
• Added a P/Invoke declaration for the Win32 SetEnvironmentVariableW function
• Implemented a fallback path to use the direct Win32 call when the .NET method fails

Fixes #11.

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