refactor(dialog)!: match native <dialog> and fix spacing

[copybara-service]

refactor(dialog)!: match native <dialog> and fix spacing

Fixes #4647
Fixes #4285

BREAKING CHANGE: See #4675 for more details.

Dialogs use the native <dialog> interface, which uses a <form method="dialog"> to set up dialog actions.

<md-dialog>
  <div slot="headline">A simple dialog</div>
  <form id="form" method="dialog" slot="content">
    This is a dialog with text content.
  </form>
  <div slot="actions">
    <md-text-button form="form" value="close">Close</md-text-button>
    <md-text-button form="form" value="ok">OK</md-text-button>
  </div>
</md-dialog>

[github-actions]

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[christophe-g]

There must be a particular reason for not having a default slot here - but I am not sure which one ?

[asyncLiz]

We add margin to the light dom slotted content, which can be customized. This requires a single root node though so we don't add margin spacing to all the internal content.

Requiring a slot attribute helps to encourage only providing a single root node.