fix: Make the selected item(s) in dropdown get read out the screen reader when focused

[anthonymuau]

This PR is to address a bug in vue-multiselect where if a user is using a screen reader, the selected state of the items are not read out when the user focuses on them.

Steps to reproduce

1. Enable a screen reader (voice-over).
2. Navigate to a page containing the vue-multiselect component.
3. Use the Tab key to move focus to the vue-multiselect component.
4. Use the Up and Down arrow keys to select an item.
5. Press the Enter key again to focus specifically on the selected item.

Expected behavior: The screen reader should announce the selected item and its selected state.

Actual behavior: The screen reader remains silent, failing to announce the selected item or its selected state.

[aelnonym]

This would be very helpful, thank you