Add support for asynchronous autoloading

[jakubbielawa]

[StewMcc]

Is this not a bug? Is there a way to work around this and still use it? 2019 ATM keeps bugging out with the warning for me 😞

[xCONFLiCTiONx]

This is for the developer but likely already knows about it. The fix to change to async: https://github.com/Microsoft/VSSDK-Extensibility-Samples/tree/master/AsyncPackageMigration

This is for the rest of us dealing with the erros/warnings until it gets fixed: https://docs.microsoft.com/en-us/visualstudio/extensibility/synchronously-autoloaded-extensions?view=vs-2019