Visual Studio Package should be marked for async load

[jaredpar]

Part of the Visual Studio performance work is having packages be fully async loaded. VsVim has an AsyncPackage now but hasn't marked it for full background load.

https://devblogs.microsoft.com/visualstudio/improving-the-responsiveness-of-critical-scenarios-by-updating-auto-load-behavior-for-extensions/

This should be pretty straight forward but need to get it done.