Add support for dialog element

[Jta26]

We are experimenting internally with moving to the dialog element, and we in the medium term need a Web <-> RN compatible dialog component that shares the same API in native.

The goal with this PR is to add minimal support for shimming between on web, and the RN component.

To set some light milestones

1. Get <html.dialog> rendering on web.
2. Add native support (rendering Modals) and shim API to match web (ref.showModal/ref.close() in react)
3. Shim the ::back-drop pseudoelement to the backdropColor prop of RN Modal

[Jta26]

Needs some further exploration.

In RN the <Modal> component roughly maps on web to the ::backdrop psuedoelement, and we may need to render an additional <View> inside the modal to match the exact behavior of <dialog> in native.

[meta-cla]

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