Block Bindings API: Create WP_Block_Bindings_Source class

[czapla]

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

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

This change was previously discussed in ​https://github.com/WordPress/gutenberg/issues/53300#issuecomment-1926572096

### 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.

[gziolo]

In 57562:

Editor: Introduce WP_Block_Bindings_Source class

Abstracts the block bindings source array into a well-defined object.

Fixes #60447.
See #60282.
Follow-up [57373].
Props czapla, santosguillamot, gziolo.

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