file:// uris not supported

UriLoader goes to the StorageFileLoader for [URIs prefixed with ms-](https://github.com/CommunityToolkit/Lottie-Windows/blob/27e3769e22ce6d081677dfdb277db929aa721156/source/Lottie/UriLoader.cs#L45), but uses the normal UriLoader (which leverages HttpClient) for everything else. This does not work for file:// URIs, which should be loaded with StorageFile.

While I can use the SetSourceAsync(StorageFile) to set the lottie in the file:// case, it would be more convenient if I could just pass in the URI regardless--and when using the StorageFile version of SetSourceAsync, the UriSource property of the LottieVisualSource remains null, so I can't use it to check if there is currently a lottie assigned.