Currently we implement few JSON-RPC data structures ourselves, e.g. here, here or here. It might be feasible to use existing solution and it'd also allow for further simplification of the protocol handling and the message flow in general.

• use appropriate types for JSON-RPC/LSP error handling flow
• factor out notification/requests (this might be helpful: Codify the methods and their arguments and result types gluon-lang/lsp-types#19)