Don't marshal Task<[]> of supported colletion items

[elringus]

.NET's JS interop currently is only able to marshal types with single-level nesting:

• [browser] Implement deeper JSImport marshaling metadata dotnet/runtime#81348

— hence we have to handle otherwise natively-supported arrays (eg, Task<byte[]>) in a special manner.

Workaround 1

Update: This workaround seems to no longer work in .NET 8.0.1, where it throws an error stating proxy for the associated method is not found.

Lift the array to object and marshal as JSType.Any:

[JSExport] [return: JSMarshalAs<JSType.Promise<JSType.Any>>]
private static async Task<object> GetArrayAsync ()
{
    await Task.Delay(1);
    return new[] { "foo", "bar", "baz" };
}

This is currently employed under the hood and doesn't require any action from end-user, but costs an extra allocation when unmarshalling from JS to C#.

Workaround 2

Wrap the call into two: first to wait for the async operation, second to get the array in a blocking manner, eg:

[JSFunction] Task OpenFile (string uri);
[JSFunction] byte[] GetOpenFileContent (string uri);

Task<byte[]> ReadFile (string uri)
{
    await OpenFile(uri);
    return GetOpenFileContent(uri);
}