refactor: 🏷️ add type for custom action

closes #269

Author: spences10

src/app.d.ts

@@ -8,3 +8,11 @@ declare namespace App {
   // interface Session {}
   // interface Stuff {}
 }
+
+// https://stackoverflow.com/questions/73025100/svelte-svelte-kit-type-custom-action-event-with-typescript
+// https://github.com/sveltejs/language-tools/blob/master/docs/preprocessors/typescript.md#im-getting-deprecation-warnings-for-sveltejsx--i-want-to-migrate-to-the-new-typings
+declare namespace svelteHTML {
+  interface HTMLAttributes<T> {
+    'on:enterViewport'?: (event: any) => any
+  }
+}