Add focus state binding example for child view

[joshuakcockrell]

This adds an example to the FocusStatePlayground that shows how @FocusState.Binding can be replaced with a normal @Binding so that @FocusState variables can be passed to child views.

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[joshuakcockrell]

I tried to run swift test but it just says error: no tests found; create a target in the 'Tests' directory?

There's not really anything that would impact tests here though so idk what's happening.

[marcprux]

I tried to run swift test but it just says error: no tests found; create a target in the 'Tests' directory?

The Showcase app doesn't have a test module. Test cases are typically part of framework projects and app projects often remove them, so you don't need to worry about them.

[marcprux]

@aabewhite Do you think we could just typealias FocusState.Binding to Binding in SkipUI? I'm unclear on why FocusState.Binding exists at all.