Fix: throw error if accordion element is not found

[Tresor-Kasenda]

This pull request includes a critical update to the Accordion class in the packages/accordion/src/accordion.ts file. The most important changes are focused on improving error handling and type safety.

Error handling improvements:

• Added a check to ensure the accordionEl element exists and throws an error if it is not found.

Type safety enhancements:

• Updated the filter function for this.items to include type annotations, ensuring item is properly typed as an object with a parentElement property.

[Johnkat-Mj]

All correct