refactor: redesign plugin system and enhance message handling

- Move plugin and tool-related classes to a dedicated `Plugin-API` module.
- Introduce a new plugin lifecycle (`onInit`, `onEnable`, `onDisable`) and a more robust `Tool` API.
- Refactor `Core` to use `PrintAdvanceMessageHandler` for improved handling of assistant messages, tool responses, and errors.
- Rename `PluginLoader` to `Loader` and migrate it to `me.zacharias.chat.plugin.loader`.
- Update build configurations and Java toolchains across modules.
- Clean up obsolete plugin annotations and interfaces in the `Core` module.

Plugin-API/src/main/java/me/zacharias/chat/plugin/Data.java

@@ -0,0 +1,9 @@
+package me.zacharias.chat.plugin;
+
+import java.io.File;
+
+public abstract class Data {
+    public abstract File getDataDictionary();
+    public abstract LoadedPlugins getLoadedPlugins();
+    public abstract File getCacheDirectory();
+}