Implement a Visual Studio extension

[alexrp]

This should simply wrap our LSP implementation.

Depends on:

• Basic LSP implementation #62

[alexrp]

As part of this, we need to figure out whatever hacks are needed to use an SDK-style VSIX project file.

Relevant links:

• https://developercommunity.visualstudio.com/t/vsix-project-with-sdk-style-csproj/1572145
• https://stackoverflow.com/questions/58917250/is-it-possible-to-write-vs-extensions-using-the-sdk-style-projects
• Templates - SDK style projects. microsoft/VSProjectSystem#257

[alexrp]

May or may not get to this in v1.0. Depends on whether the VS team gets around to fixing some of the silliness here, like the lack of official support for SDK-style projects.

[alexrp]

We can at least start the process now: https://github.com/microsoft/VSExtensibility/blob/main/New_Extensibility_Model/Samples/DialogSample/DialogSample.csproj

Notably, we can't yet produce a VSIX (see e.g. microsoft/VSExtensibility#242), but that can come later.