[feat] <rh-dialog>: switch to native <dialog> element

[brianferry]

Description

<rh-dialog> should be set up to use the native <dialog> element if possible and decouple from using the PfModal Base Class.

Suggested solution

Switch over the code and the styles from PfModal to be in the <rh-dialog> element first of all, then convert it to a native <dialog> html element inside the shadow dom.

Example API

Same slots and api as currently exists, just updated internals

Additional context

No response