Request dispatcher's return value determines response

Request dispatcher's return value determines response

No more jsonrpc-reply.

* eglot.el (eglot-handle-request window/showMessageRequest):
Simplify.
(eglot--register-unregister): Simplify.
(eglot-handle-request workspace/applyEdit): Simplify.
(eglot--apply-text-edits): Signal a jsonrpc-error.
(eglot--apply-workspace-edit): Simplify.

* jsonrpc-tests.el (jsonrpc--with-emacsrpc-fixture): Don't
jsonrpc--reply.

* jsonrpc.el (jsonrpc-error, jsonrpc-connection, jsonrpc-request):
Improve docstring.
(jsonrpc-error): Polymorphic args.
(jsonrpc--unanswered-request-id): Remove.
(jsonrpc--connection-receive): Rework and simplify.
(jsonrpc-reply): Simplify.