[Language Server] Support for modern versions of LSP

[tintoy]

This is the server-side (i.e. language server) portion of the work in msbuild-project-tools-vscode#177.

When MSBuild Project Tools was originally built, the LSP hosting APIs provided by OmniSharp LSP looked very different than they do today.

I suspect there will be significant effort involved in this work, but lack of modern LSP support is holding us back (even limiting us in what version of the LSP client libraries we can use in our extension) so it has to be done.