Enable commands in all environments

- Many command-related fixes and improvements
- Added test environment for easier development

test-fabric/src/main/java/eu/midnightdust/fabric/example/MLExampleFabric.java

@@ -0,0 +1,11 @@
+package eu.midnightdust.fabric.example;
+
+import eu.midnightdust.fabric.example.config.MidnightConfigExample;
+import net.fabricmc.api.ModInitializer;
+
+public class MLExampleFabric implements ModInitializer {
+    @Override
+    public void onInitialize() {
+        MidnightConfigExample.init("modid", MidnightConfigExample.class);
+    }
+}