Basic LSP implementation

[alexrp]

The syntax/semantic analysis APIs should now be sufficient for a basic LSP implementation. We don't need to be super ambitious here - just performing semantic highlighting would be a great first step.

Depends on:

• Support a celerity.json project configuration file #30
• Switch to Draco.Lsp for the language server #86

[alexrp]

e2dbe09 implemented syntax classification.

Now we just need to hook up an LSP handler to our workspace and classification APIs.

[alexrp]

Depends on:

• Make workspace APIs more flexible #78
• Introduce a concept of project solutions #82