Timer: make the interval property mandatory

A default of 0 is not a great default because that's way too often

internal/compiler/passes/lower_timers.rs

@@ -42,6 +42,14 @@ fn lower_timer(
         return;
     }
 
+    if !timer_element.borrow().is_binding_set("interval", true) {
+        diag.push_error(
+            "Timer must have a binding set for its 'interval' property".into(),
+            &*timer_element.borrow(),
+        );
+        return;
+    }
+
     // Remove the timer_element from its parent
     let old_size = parent_element.borrow().children.len();
     parent_element.borrow_mut().children.retain(|child| !Rc::ptr_eq(child, timer_element));

internal/compiler/tests/syntax/elements/timer2.slint

@@ -11,4 +11,12 @@ export component Def {
     //       ^error{Timer cannot be directly repeated or conditional}
 
     if false: Abc {}
+
+    Timer {
+//  ^error{Timer must have a binding set for its 'interval' property}
+        running: false;
+    }
+
+    Timer {}
+//  ^error{Timer must have a binding set for its 'interval' property}
 }