Site Editing block placeholders

[jameskoster]

A couple of recent PRs have sought to tidy up placeholder states for some of the site editing blocks:

• Remove extra styling around "Post content" placeholder. #35243
• Try: Site Logo placeholder tweaks #35397

It would be good to do this holistically across all of the site editing blocks.

To clarify, these placeholders are for situations in which the blocks have no data to reference.

Here's how they look currently:

There are several inconsistencies to address here. We have different content schemas, styling, image handling and so on.

For the text-based blocks, it's probably adequate to let the theme stylesheet handle their appearance. So let's remove any styling like padding, borders, font-size etc.

In regards to their content, I'm inclined to say that echoing the block name is probably the simplest and most intuitive approach for now. So instead of "An example title" and "no date" we'd have "Post title" and "Post date". This feels like a good foundation from which we can potentially do something more elaborate in the future ("Hello Dolly" for placeholder content?).

The new dashed outline applied to the site logo works well, so perhaps we can try that for the Featured Image as well.

• Update styling: Update site editor block placeholder styling #35513
• Update content: Unify theme block placeholder content #35517
• Featured Image: Apply Site Logo placeholder treatment to Featured Image block #36136

[noisysocks]

Closing this out in favour of #36136 which is more specific/actionable.