Improve assembly resolve behavior

[jasongin]

Fixes: #286

• Update the resolving event to separate the functionality of resolving an assembly from dynamically exporting an assembly's types.
• Search .NET system directories (Microsoft.NETCore.App, Microsoft.AspNetCore.App, and Microsoft.WindowsDesktop.App if present) when resolving assemblies, before raising the resolving event.
• Search directories of already-resolved assemblies when an assembly is not resolved by the resolving event listener.
• Defer dynamic export of System.Runtime and System.Console assembly types until the first time the dotnet.System property is accessed from JavaScript. This significantly improves startup time for scenarios that do not use dynamic export.
• Bump version to 0.8