Feat: Add blocks.syncType filter for sync toggle

[badasswp]

What?

Closes #71459

This PR adds a custom filter blocks.syncType to the wp_block pattern post type.

Why?

At the moment, when a new pattern is created, the synced toggle is forced into ON mode. This feels counter-intuitive for users who would prefer that this is OFF by default so that they can decide to toggle it ON if they wish so. Plus, there's no way of customising this whatsoever because the toggle value is hard-coded by default to undefined.

How?

This PR introduces a filter blocks.syncType to the wp_block pattern post type so that users can be able to correctly set the toggle's default state when a new pattern is created.

[github-actions]

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[github-actions]

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