api metadata: Allow typed container information in api functions

Adapt gendeclarations.lua/msgpack-gen.lua to allow the `ArrayOf(...)` and
`DictionaryOf(...)` types in function headers. These are simple macros that
expand to Array and Dictionary respectively, but the information is kept in the
metadata object, which is useful for building clients in statically typed
languages.

src/nvim/api/private/defs.h

@@ -10,6 +10,11 @@
 #define OBJECT_INIT { .type = kObjectTypeNil }
 #define REMOTE_TYPE(type) typedef uint64_t type
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+  #define ArrayOf(...) Array
+  #define DictionaryOf(...) Dictionary
+#endif
+
 // Basic types
 typedef struct {
   char msg[256];