feat: custom margin top scale for background

[bryantbrock]

Allow moving the scaled background down some to look better in PWA or mobile apps (using capacitor).

Pass in a value to the root backgroundMarginTop={75} and see the background page shift down.

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[bryantbrock]

FYI I'm referencing this very commit in a production app, testing looks great. Don't take my word for it though.

[RicardoDalcin]

Hey! Any updates regarding this PR?

I have the exact same issue as @bryantbrock using Vaul in a Capacitor app.

If the idea is not to overcomplicate the API with more props, it is also feasible to change the direct use of env(safe-area-inset-top) to a CSS variable that defaults to it, so that it can at least be overridden with CSS.