Fix missed renames in #22632 (#23688)

Fix a bug where a GPUI macro still used `ModelContext`
Rename `AsyncAppContext` -> `AsyncApp`
Rename update_model, read_model, insert_model, and reserve_model to
update_entity, read_entity, insert_entity, and reserve_entity

Release Notes:

- N/A

crates/gpui_macros/tests/render_test.rs

@@ -0,0 +1,7 @@
+#[test]
+fn test_derive_render() {
+    use gpui_macros::Render;
+
+    #[derive(Render)]
+    struct _Element;
+}