Fixed generated properties within regions

[Sergio0694]

Fixes #4225

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

Compilation fails If the Microsoft MVVM Toolkit source generator attribute [ObservableProperty] is in a #region block.

What is the new behavior?

The generated code is now correct when regions are used.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• New component
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
  • If control, added to Visual Studio Design project
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• New major technical changes in the toolkit have or will be added to the Wiki e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

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