Investigate building a language server for proto files for GRPC

[JunTaoLuo]

This would be in addition to syntax highlighting and grammar. This would allow us to have intellisense-like language support such as auto-completion for imported message types.

[juwens]

As first priority: please add squiggly error highlighting. Currently to spot an error, we need to compile the project, look in the build output and spot the proto error.