make internals public but behind spi

[stevenbrix]

this pr makes the internals public, since it doesn't affect the export count on debug builds and puts them all behind @_spi(WinRTInternal). this will make it easier to fall back to something like this for accessing the throwing set/get of properties

let abi: __ABI_Microsoft_UI_Xaml.IFoo = try swift.thisPtr.QueryInterface()
let foo = try abi.get_Foo()