[Feature]: Comprehensive Set of Placeholders

### 🚀 Description

Similar to the Placeholder engine introduced in Simple RPC v4, I want to do the same for SDLink.

This means that placeholders will change from the ugly `%PLACEHOLDER%`, to `{{placeholder}}` or {{placeholder.subvalue}}`.

Additionally I also want to support nicer color placeholders in the config for MiniMessage.

The new placeholders should be universal across config and embeds, and not different like they currently are.

For placeholder requests, please comment the placeholder you would like, how it's expected to work and where it should be usable.

### 💡 Use Case

_No response_