Improve WPF example

[jasongin]

• Fix some issues in the TS generator (particularly in SymbolExtensions) that prevented generating type definitions for WPF assemblies. WPF has particularly complex type graphs and use of generics that uncovered some tricky edge cases.
• Allow the dotnet.load() API to load native libraries in addition to .NET assemblies. This enables explicitly loading native libraries when automatic resolution fails.
• Enrich the WPF example to include a custom Window subclass with a WebView2 control.
  • For now it needs to explicitly load some dependencies. I'll try to make that simpler / more automatic in the future.