[Extension] Support for modern versions of LSP

[tintoy]

This is the client-side (i.e. VSCode extension) portion of the work in tintoy/msbuild-project-tools-server#114.

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.

After upgrading the language server, we will want to upgrade to the current version of vscode-languageclient, and then investigate any improvements (such as those that can support #175) that can be made to the way the extension (and VS Code) interact with the language server.