JavaNativeModule: Fixed Method selection for Functional interfaces

Background: We only checked public nondefault, that matched for public statics as well.
Now checks for public nodefault abstract

Also TODO: Functional Type may not be part of functional Method's signature

JavaIntegration/src/main/java/org/openzen/zencode/java/JavaNativeModule.java

 	private StoredType loadFunctionalInterface(TypeVariableContext loadContext, Class<?> cls, AnnotatedElement[] parameters) {
 		Method functionalInterfaceMethod = getFunctionalInterfaceMethod(cls);
 		TypeVariableContext context = convertTypeParameters(cls);
+		//TODO: This breaks if the functional interface type appears in the method's signature
 		FunctionHeader header = getHeader(context, functionalInterfaceMethod);
 
 		Map<TypeParameter, StoredType> mapping = new HashMap<>();
 
 	private Method getFunctionalInterfaceMethod(Class<?> functionalInterface) {
 		for (Method method : functionalInterface.getDeclaredMethods()) {
-			if (Modifier.isPublic(method.getModifiers()) && !method.isDefault())
+			if (Modifier.isPublic(method.getModifiers()) && Modifier.isAbstract(method.getModifiers()) && !method.isDefault())
 				return method;
 		}