/// A single JSON-RPC message exchanged between the editor's LSP client and a
/// language server.
///
/// The message is the serialized JSON-RPC object as a string, without
/// `Content-Length` framing -- the `@codemirror/lsp-client` transport is
/// message-based, so each `LspMessage` is exactly one JSON-RPC object, e.g.
/// `{"jsonrpc":"2.0","id":1,"method":"initialize","params":{}}`.