feat: context api overhaul

packages/core/tests/diff_component.rs

@@ -69,7 +69,7 @@ fn component_swap() {
             LoadTemplate { name: "template", index: 0, id: ElementId(4) },
             ReplacePlaceholder { path: &[1], m: 3 },
             LoadTemplate { name: "template", index: 0, id: ElementId(5) },
-            AppendChildren { m: 2 }
+            AppendChildren { m: 2, id: ElementId(0) }
         ]
     );