feat(Scripts/Commands): implement new command: .inventory count (#9139)

- Closes #8845

data/sql/updates/pending_db_world/rev_1636656798364802400.sql

@@ -0,0 +1,6 @@
+INSERT INTO `version_db_world` (`sql_rev`) VALUES ('1636656798364802400');
+
+DELETE FROM `command` WHERE `name` IN ('inventory','inventory count');
+INSERT INTO `command` VALUES
+('inventory',1,'Syntax: .inventory $subcommand \nType .inventory to see the list of possible subcommands or .help inventory $subcommand to see info on subcommands'),
+('inventory count',1,'Syntax: .inventory count $playerName or $plaerGuid \nCount free slots in bags divided into different bag types.');