refactor(mappings)!: mapblock_fill_dict() use API Dictionary (#20020)

This introduces the following breaking changes:
- nvim_get_keymap now always returns a LuaRef object as "callback" for a
  Lua mapping regardless of how it is called. The LuaRef object can be
  called from Lua and Vim script, but is lost over RPC.
- maparg() now returns a Funcref instead of a ref number as "callback"
  for a Lua mapping. The Funcref can be called from Lua and Vim script,
  but is lost over RPC.

This may also make nvim_get_keymap faster, but make maparg() slower.

src/nvim/api/buffer.c

@@ -945,7 +945,7 @@ Integer nvim_buf_get_changedtick(Buffer buffer, Error *err)
 /// @param[out]  err   Error details, if any
 /// @returns Array of |maparg()|-like dictionaries describing mappings.
 ///          The "buffer" key holds the associated buffer handle.
-ArrayOf(Dictionary) nvim_buf_get_keymap(uint64_t channel_id, Buffer buffer, String mode, Error *err)
+ArrayOf(Dictionary) nvim_buf_get_keymap(Buffer buffer, String mode, Error *err)
   FUNC_API_SINCE(3)
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
@@ -954,7 +954,7 @@ ArrayOf(Dictionary) nvim_buf_get_keymap(uint64_t channel_id, Buffer buffer, Stri
     return (Array)ARRAY_DICT_INIT;
   }
 
-  return keymap_array(mode, buf, channel_id == LUA_INTERNAL_CALL);
+  return keymap_array(mode, buf);
 }
 
 /// Sets a buffer-local |mapping| for the given mode.