Improve build-time support for UserControl about react-native-xaml

[asklar]

Design A: split built logic

• Produce separate files for XAML and the custom WinMDs. The custom WinMD codegen would need to be part of the app
• This poses a problem since the custom codegen still needs to get linked into ReactNativeXaml, so we end up reaching into the app's project via msbuild props
• But also, RNX needs to have a winmd reference to the custom winmd so that cppwinrt can process it, so this would effectively still require modifying RNX under node_modules. An alternative would be to reuse the app's cppwinrt generated headers but this is very fragile (e.g. different versions of cppwinrt between the app and RNX would cause build breaks).
• moreover this complicates the codegen'd TS since type needs to be a set of known strings (known at codegen time). An alternative is to have a synthetic property on UserControl like userControlType that is just a string (as opposed to type which is a string union); we lose intellisense/type safety with this so not ideal either.
• I played with this but I'm not sure this is a good way forward. Branch here: https://github.com/asklar/react-native-xaml/tree/buildLogic

Design B: view manager generator

• Codegen the whole view manager, not just the metadata. Then the app project just gets everything in one package
• Apps that only need XAML can use the view manager from the package, apps that need custom stuff will use the generated one
• It would make sense to split codegen into its own npm package, and have the XAML VM be wholly generated by it
• This could make it trickier to iterate on the VM (since the experience isn't great for T4 files, no intellisense, etc., it's just text)
• This is closer to what is possible today; you can run codegen and it will update the code in node_modules, and then you can copy the resulting RNX project into your project and get rid of the RNX npm package dependency