Block Bindings API: Create WP_Block_Bindings_Source class

[michalczaplinski]

This PR refactors the array of Block Binding sources into its own WP_Block_Bindings_Source class.

This change was previously discussed in WordPress/gutenberg#53300 (comment)

Reasons for this change

• Having a well-defined class for the sources makes it clearer what types are expected to be passed.
• It makes the implementation more flexible for future changes.

Trac ticket: https://core.trac.wordpress.org/ticket/60447#ticket

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Props czapla, gziolo.

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Co-authored-by: michalczaplinski <czapla@git.wordpress.org>
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[gziolo]

This is shaping up nicely. I really like how the new class improves the readability of the code and how the refactoring brought attention to additional validation for the properties passed.

I left my feedback for potential improvements.

[gziolo]

Nit: It would be possible to inject only get_value_callback here, but we can change that with the refactoring. Overall, that makes perfect sense to use a static variable 👍🏻

[gziolo]

Committed with https://core.trac.wordpress.org/changeset/57562.