Gutenberg: Convert to Blocks Prompt [3]

[gwwar]

In this task lets add a prompt to ask folks to convert the post when opening a post made in the Classic editor in Gutenberg. (Gutenberg has this option under the classic block, but we'll want to surface this functionality).

This was previously implemented in #29790, but should be added to our latest integration.

To outline behavior requirements:

• Opening a post with a single Classic block displays the prompt
• Clicking on Convert to Blocks will convert the classic blocks
• Folks may undo this action using normal Editor undo

Implementation wise, this may be done at /block-editor only, or at both /wp-admin and /block-editor. Depending on what folks choose we may need an additional Jetpack PR, or use core dialog components.

See also prior work in p7jreA-22i-p2 and discussion in p58i-7OD-p2
See PCYsg-l4k-p2 and PCYsg-l4k-p2 for how the integration works

[mtias]

It would be good to assure users that this can be undone if something doesn't look right.