fix(ui5-multi-input): fire change on enter press inside of form

[StefanDimitrov04]

FIXES: #12934

[ui5-webcomponents-bot]

🧹 Preview deployment cleaned up: https://pr-12936--ui5-webcomponents.netlify.app

[nnaydenow]

We need to validate the following use cases:

1. Form submission with empty input + {ENTER_KEY}
2. Form submission with typed input + {ENTER_KEY}
3. Form submission with typeahead input + {ENTER_KEY}
4. Form submission when focus is inside the dropdown + {ENTER_KEY}

Expected behavior:

1. Submit the form.
2. Fire a change event, then submit the form.
3. Fire a change event to confirm the autocomplete suggestion; if an additional {ENTER_KEY} is pressed, submit the form.
4. Fire a change event to confirm the selection; if an additional {ENTER_KEY} is pressed, submit the form.