Implement a Visual Studio extension

[alexrp]

This should simply wrap our LSP implementation.

Depends on:

• [ ] #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
• https://github.com/Microsoft/VSProjectSystem/issues/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.

[alexrp]

Started working on this in the work/vs branch. It currently builds the extension assembly on all platforms, but only produces the VSIX on Windows.

Things to be done:

• Figure out if we can make VSIX creation work on Linux/macOS.
• Figure out how to publish the extension from the Cake script, similar to the VS Code extension.
• Figure out how to make the development/debugging process as easy as possible in VS.
• Maybe separate out the build steps for the VS extension from the core projects in the Cake script.

[alexrp]

The VS extensibility packages are now available on NuGet, so some of the hacks in the branch can now be removed.

Still need to see if there's a way to create the VSIX on Linux/macOS.