[confirm-dialog] Update to not use dialog extension internally and use mixins instead

[web-padawan]

Motivation

Due to historical reasons, vaadin-confirm-dialog uses vaadin-confirm-dialog-dialog element under the hood. This can be problematic for the native popover refactoring in terms of additional nested shadow root.

Proposed solution

• Extract some mixins from DialogBaseMixin - e.g. a separate mixin for width and height
• Update vaadin-confirm-dialog to use these mixins internally instead of the dialog extension
• Place vaadin-confirm-dialog-overlay inside of the vaadin-confirm-dialog shadow root
• Re-implement some dialog logic e.g. noCloseOnEsc is quite trivial (a single event listener)