Editor: Hide Classic Block from inserter

[tyxla]

Trac ticket: https://core.trac.wordpress.org/ticket/65166

This PR brings the changes from the following Gutenberg PR to core:

WordPress/gutenberg#77911

Description

We're hiding the Classic block in the block editor inserter by default.

We're also introducing a new wp_classic_block_supports_inserter filter that allows overriding that behavior globally or per post.

See: WordPress/gutenberg#74242

Testing

• Start writing a new post
• Verify you don't see the Classic block in the inserter.
• Open an existing post that contains a classic block (or a <!-- wp:freeform --> block) and confirm the Classic block renders correctly and is editable.
• Put this in your code: add_filter( 'gutenberg_classic_block_supports_inserter', '__return_true' );. Alternatively, you can use this plugin to test it.
• Verify you see the Classic block in the inserter.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props tyxla, mukesh27.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.