feat(plugins): Added message read/update methods to the Channel API. (#6650)

This PR adds methods to read and update messages in a channel. Specifically, it adds:
 - message_snapshot
 - last_message
 - replace_message/replace_message_at
 - clear_messages
 - find_message_by_id
 - has_messages
 - count_messages


Reviewed-by: Mm2PL <mm2pl+gh@kotmisia.pl>

docs/chatterino.d.ts

@@ -63,6 +63,20 @@ declare namespace c2 {
             override_flags?: MessageFlag | null
         ): void;
 
+        message_snapshot(n_items: number): Message[];
+        last_message(): Message | null;
+        replace_message(message: Message, replacement: Message): void;
+        replace_message(
+            message: Message,
+            replacement: Message,
+            hint: number
+        ): void;
+        replace_message_at(index: number, replacement: Message): void;
+        clear_messages(): void;
+        find_message_by_id(id: string): Message | null;
+        has_messages(): boolean;
+        count_messages(): number;
+
         is_twitch_channel(): boolean;
 
         get_room_modes(): RoomModes;