Use Jena's initialization mechanism for subsystems

jena/src/main/resources/META-INF/services/org.apache.jena.sys.JenaSubsystemLifecycle

@@ -0,0 +1 @@
+eu.ostrzyciel.jelly.convert.jena.riot.JellySubsystemLifecycle

jena/src/main/scala/eu/ostrzyciel/jelly/convert/jena/riot/JellyLanguage.scala

@@ -20,7 +20,8 @@ object JellyLanguage:
   /**
    * Register the Jelly language and formats in Jena.
    *
-   * This method is idempotent and should be called automatically when the class is loaded.
+   * This method is idempotent and should be called automatically when Jena is initialized.
+   * See: https://jena.apache.org/documentation/notes/system-initialization.html
    * However, you may also want to call this manually if Jena doesn't load the language automatically.
    */
   def register(): Unit = this.synchronized {

jena/src/main/scala/eu/ostrzyciel/jelly/convert/jena/riot/JellySubsystemLifecycle.scala

@@ -0,0 +1,17 @@
+package eu.ostrzyciel.jelly.convert.jena.riot
+
+import org.apache.jena.sys.JenaSubsystemLifecycle
+
+/**
+ * Subsystem lifecycle for registering the Jelly language in Jena.
+ * See: https://jena.apache.org/documentation/notes/system-initialization.html
+ */
+class JellySubsystemLifecycle extends JenaSubsystemLifecycle:
+  override def start(): Unit = JellyLanguage.register()
+
+  override def stop(): Unit = ()
+
+  // "Application code should use level above 500."
+  // Not clear whether the plugins should use > 500 or < 500...
+  // Let's use 501, this should make Jelly load before the other application subsystems.
+  override def level() = 501