Introduce option to auto-convert freeform content upon opening the editor

[mtias]

It's about time we start looking into further smoothing out the transition points from classic content to blocks. We have some pending refinements to do on validation and generally being more permissive about attribute spread, but there are also some steps we can start formalizing before we consider ways to do bulk conversion of content in the future.

One of these steps would be to introduce a configuration setting to WordPress so that the editor attempts to auto-convert freeform content into blocks the moment you open the editor, instead of handing it over to the classic block and requiring clicking the "convert to blocks" button. This should be a reversible action, for the most part. I'd imagine a notice triggering "content was updated" with the option to undo. We could go further and use this same setting to stop loading the classic block (and all its dependencies) from the block library.

I imagine a few extra requirements and considerations:

• Probably the best start is to make it a wp-config constant: AUTO_CONVERT_TO_BLOCKS, maybe supporting a definition that can include specific post types so you can restrict auto-conversion to pages but leave it off from posts, etc.
• Obviously would be set to off by default and opt-in.
• Maybe also don't allow the constant to trigger any behaviour if WP_POST_REVISIONS is also false, as we'd want to limit any operation that is not easily retrievable from prior state when something goes wrong.

Any other considerations I might be missing?

[github-actions]

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[github-actions]

This issue has gone 30 days without any activity.