Request/Notification types should be discoverable with implict-member-expression

[rintaro]

Requests should be constructible with implicit member expressions when the expected type is RequestType.

func handle<Request: RequestType>(request: Request) async -> Request.Response { .... }

func test() {
  handle(.<COMPLETE>)
}

To do that,

• Introduce LSPRequestType that inherits RequestType
• Every LSP request should inherit LSPRequestType
• All LSP request handling function should accept LSPRequestType instead of RequestType
• For every request type, declare extension LSPRequestType where Self == XXXRequest to declare a static factory func

I.e.

public protocol LSPRequestType: RequestType {}
public struct ApplyEditRequest: LSPRequestType, Hashable { ... }

extension LSPRequestType where Self == ApplyEditRequest {
  public static func applyEdit(label: String? = nil, edit: WorkspaceEdit) -> Self {
    .init(label: label, edit: edit)
  }
}

This applies to LSP requests and notifications, and BSP requests and notifications

[rintaro]

Decided not to this.
But having separate base types for LSP/BSP each will improve the discoverability. ( #18 )