Add block visibility support

[t-hamano]

Trac ticket: https://core.trac.wordpress.org/ticket/64061

Testing Instructions

• Create a new post.
• Create content by using the following HTML:

  <!-- wp:paragraph {"metadata":{"blockVisibility":false}} -->
  <p>This text should NOT be visible on the front end.</p>
  <!-- /wp:paragraph -->
  
  <!-- wp:paragraph {"metadata":{"blockVisibility":true}} -->
  <p>This text should be visible on the front end.</p>
  <!-- /wp:paragraph -->
  
  <!-- wp:paragraph-->
  <p>This text should be visible on the front end.</p>
  <!-- /wp:post-content -->

• Confirm that the first block of three paragraphs does not render anything on the frontend.

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Props wildworks, mukesh27.

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[mukeshpanchal27]

IMO, we should move these checks to the top. If the attribute is already set, we can return early and avoid registering the type unnecessarily. WDYT?

[t-hamano]

we should move these checks to the top

I understand the intention, but I think we should check the block support first. Otherwise, the block may return an empty string even though it doesn't support blockVisibility.