Convert package to AsyncPackage

"As part of ongoing performance efforts to guarantee a faster startup and solution load experience for all users, Visual Studio will change how auto loaded packages work during startup and solution load scenarios.

All packages that utilize auto load rules will have to support background load and implement asynchronous initialization. The IDE will no longer synchronously auto load packages in any UI context, including rule-based UI contexts."

https://blogs.msdn.microsoft.com/visualstudio/2018/05/16/improving-the-responsiveness-of-critical-scenarios-by-updating-auto-load-behavior-for-extensions/

https://docs.microsoft.com/en-us/visualstudio/extensibility/how-to-use-asyncpackage-to-load-vspackages-in-the-background?view=vs-2017