Additional slot for notifications in ModalDialog

[devmount]

Is your feature request related to a problem? Please describe.
In case an error message needs to be displayed directly in the ModalDialog component, it can only be shown in the modal body as of now.

Describe the solution you'd like
An additional slot for showing messages to the user to have a centralized/unified way to do so.

Describe alternatives you've considered
We could already integrated the NoticeBar component and only forward the message data as properties.

Additional context
thunderbird/appointment#1564 (comment)

[devmount]

Tagging @laurelterlesky since we need design approval

[laurelterlesky]

We're working on this - will come back with the modal component update to include the notifications area.