feat: Adds ModMail cog with core setup

Introduces a private, forum-based ModMail system for relaying user DMs to staff threads, enhancing moderation and support through persistent message storage.

Relies on 'rich' requirement for better formatting; includes metadata for easy installation and help integration.

modmail/info.json

@@ -0,0 +1,16 @@
+{
+  "author": [ "unstableCogs" ],
+  "install_msg": "Thank you for installing the ModMail cog! Please use `[p]help ModMail` for setup commands.",
+  "name": "ModMail",
+  "short": "A private, forum-based ModMail system.",
+  "description": "A comprehensive ModMail system that relays user DMs to a private forum channel for staff to review and respond to. This serves as the core for all ticket-based interactions.",
+  "tags": [
+    "modmail",
+    "moderation",
+    "support",
+    "tickets",
+    "utility"
+  ],
+  "requirements": [ "rich" ],
+  "end_user_data_statement": "This cog persistently stores user IDs and message content from ModMail threads for moderation and support history."
+}