api: rename "msgpack_rpc/defs.h" to "api/private/dispatch.h" and use the header generator.

src/nvim/api/private/dispatch.h

@@ -0,0 +1,21 @@
+#ifndef NVIM_API_PRIVATE_DISPATCH_H
+#define NVIM_API_PRIVATE_DISPATCH_H
+
+#include "nvim/api/private/defs.h"
+
+/// The rpc_method_handlers table, used in msgpack_rpc_dispatch(), stores
+/// functions of this type.
+typedef struct {
+  Object (*fn)(uint64_t channel_id,
+               uint64_t request_id,
+               Array args,
+               Error *error);
+  bool async;  // function is always safe to run immediately instead of being
+               // put in a request queue for handling when nvim waits for input.
+} MsgpackRpcRequestHandler;
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "api/private/dispatch.h.generated.h"
+#endif
+
+#endif  // NVIM_API_PRIVATE_DISPATCH_H