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.

Display/build.gradle

@@ -24,4 +24,10 @@ jar{
     manifest {
         attributes 'Main-Class': 'me.zacharias.chat.display.Main'
     }
+}
+
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(javaVersion)) // Set Java version
+    }
 }