Add Support for Visual Studio 2019

Visual Studio 2019 will apparently be dropping support for synchronous packages, so a simple update to the targets was not a long-term solution. The project has been converted to an AsyncPackage, but because of the reliance on DTE, the extension mostly runs on the main thread and still blocks the UI. Visual Studio will give a warning about the extension, but there's not much that can be done short of rewriting the entire thing without using the DTE interface, so disabling the warnings/reports about the extension will have to suffice.