Optionally remove margins around Dialog

[rolfsmeds]

Describe your motivation

There are at least two common UI patterns that call for a dialog (or similar overlay) that sits flush with the edges of the browser viewport:

• Full-screen modals on mobile (these are different from "views" in that they don't require navigating away from the current view, and they typically obscure the app's navigation controls). Full-screen modals are often used on mobiles to maximize utilization of the small viewport.

• Side panel overlays (modal or not) on bigger viewports, e.g.:
  

The Dialog component currently doesn't make it easy to achieve due to the --lumo-space-m margin that always surrounds them:

Describe the solution you'd like

In addition to a positioning API as proposed in #1060 and vaadin/flow-components#1173, it would be nice to also have a way to disable the margin.

This could be either through a margin API e.g. setMargin(boolean) / no-margin, or a theme variant.

Describe alternatives you've considered

No response

Additional context

No response