Add support for iOS' safe area environment variables to prevent the navbar from being hidden behind the iPhone's home bar.

It works using theofficial Webkit guide, which was originally made after the introduction of the iPhone X with iOS 11.
It exposes theenv() CSS function with 4 variables (safe-area-inset-{top,left,right,bottom}),requiringviewport-fit=cover in theviewport meta tag to get rid of natural viewport margins(edit: apparently not needed from my testing, removed it - probably only required for the left/right variants?).

These environment variables are smartly populated by the system based on the current Safari layout, giving them a value granularly as needed.

On desktop, theenv() function is (supposedly) undefined, invalidating the CSS lines using it, leading to no repercussions on desktop browsers or other mobile browsers. I've tested it on Arc / Chrome desktop in responsive mode and noticed no impact.

Demo

Before(old Safari layout/in-app browser)	After(old Safari layout/in-app browser)

After(old Safari layout/in-app browser - "unscrolled" state)	After(new Safari layout - not impacted)

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