Added searchable isPresented support

[fhasse95]

This PR adds support for controlling search presentation with the searchable(text:isPresented:) modifier. The bound isPresented value can now show and hide the search field programmatically, and user-driven search focus or cancellation updates the binding.

Related PRs:

• Added searchable isPresented support skip-fuse-ui#141
• Added searchable isPresented support skipapp-showcase#120

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