Implement a LSP server so Visual Studio Code and nvim and others can have syntax highlighting

[paulsmith]

This doesn't need to implement all the Language Server Protocol capabilities to be useful. I think just having code completion and syntax highlighting would be 95% of what we would want.

[paulsmith]

References:

• Overview
• Spec
• VS Code extension guide

[paulsmith]

Will need to make the Pushup parser more error-tolerant, since most changes will leave the current document in a broken state.