[Feature]: Support Advanced Peripherals Hidden Messages

### 🚀 Description

When `Advanced Peripherals` is installed, prefixing a message with `$` will make it hidden, and it will not show up in normal chat. Currently, these hidden messages still show up in the linked discord channel. sdlink should filter them out.

### 💡 Use Case

These hidden messages are useful for making custom commands using computercraft, and it's annoying that using them spams the discord chat

No file or test is named. Start at the chat-to-Discord forwarding entry point and trace how Minecraft messages are handled when Advanced Peripherals is installed; done means messages prefixed with $ are excluded from the linked Discord channel while normal messages continue to appear.

Written by the indexing model from the issue text.