MDL-78217 grade: Improve front-end grade item weight calculations

This change enhances the user experience in the grade report grader by
moving grade item weight calculations to the front-end. Previously,
these calculations were done on form submission, resulting in slower
performance and less interactivity.

With this improvement, grade item weights are now calculated dynamically
in the browser as users make changes, providing instant feedback and a
more responsive interface. This change improves the usability and
efficiency of the grade report grader.

This is only applicable to natural aggregation type.