Navigator Screen: warn if path doesn't follow a URL-like scheme

[ciampo]

What?

Add a runtime check in NavigatorScreen to warn consumers when the path prop doesn't follow the expected format

Why?

When the expected format is not used, Navigator doesn't behave as expected (see, for example, #65109). A runtime check is one more way that we can help consumers of Navigator to use the component as expected.

How?

If the path doesn't start with /, output a warning to console.

Testing Instructions

In dev mode, in the editor try changing the path of a NavigatorScreen component and remove the leading / — a warning should be printed to console.

The new unit test should pass.

[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>

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[Mamaduka]

Thank you, @ciampo!

[ramonjd]

I have been warned! LGTM