Add warning for missing blocks

[brandonpayton]

Description

This PR adds a dedicated fallback block to handle the cases where a block in our content no longer exists.

Fixes #7811.

How has this been tested?

Manually tested using local custom blocks, deactivating and reactivating the plugin. I observed the missing blocks fallback when the custom block plugin was deactivated, and I observed the original block type once I reactivated the plugin.

The plan is to add automated tests as well.

Screenshots

Types of changes

• Adds a core/unknown block to use instead of core/freeform for fallback.
• The parser now saves the original block name and attributes when a block is missing and is loaded by the fallback block instead.
• When the fallback block supports preserveOriginalContent, the serializer preserves its original content.

Remaining

• Get copy reviewed and corrected.
• Write unit tests.
• Update or fixes for existing tests .
• Determine how to preserve nested content or defer until after the Try Gutenberg release.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[brandonpayton]

🚨 This PR is broken. I thought the fallback block was simply for unknown blocks, but it is for "classic" content as well, which means this PR currently treats classic content as an unknown block.

Time to review this and determine next steps.