Introduce unified safeArea property

[rigor789]

Since there is a need for multiple safe area modifications I came to this thought.
Would it make sense if there was a single safeArea property that accepts types?

Here is an example in the form of xml:

<GridLayout iosSafeArea="normal"/><!-- default, usually no need to set -->
<GridLayout iosSafeArea="overflow"/>
<GridLayout iosSafeArea="ignore"/>

Of course, this would be a problem for users that already use existing overflow property.

Originally posted by @dimitrisrk in NativeScript/NativeScript#9092 (comment)

[rigor789]

@dimitrisrk I like it - however I wonder if we should just use safeArea="..." opening up the possibility to implement on Android in the future (if required).

Either way it'd be a breaking change - but the migration would be easy - so I'd say we could go for it in 8.0

Originally posted in NativeScript/NativeScript#9092 (comment)

[rigor789]

Another thought: we could introduce this in 8.0 and alias the old iosOverflowSafeArea property to it and mark it deprecated until the next major, giving people time to refactor without breaking anything.

[rigor789]

Possible implementation for Android: https://developer.android.com/reference/androidx/core/view/WindowInsetsCompat