Add UI for blockType and postType

Page patterns are companions of blocks that together form a page layout, or design. They are leveraged within StartPageOptionsModal, as starter patterns you can use to quickly add whole page designs.

Registering a page pattern includes setting the blockType to core/post-content and setting the postType to `page. Here's how Twenty Twenty-Four does it. Block types are an array of blocks that the pattern is intended to be used with.

While we do have some behind-the-scenes workarounds here, for indicating core header and footer patterns, there is no interface.

I'd like to propose a method for registering a pattern's block type, and supported post type (post and page) within the pattern creation experience of the directory. That way a theme author can build headers, footers, and page designs in the pattern directory, and add them to themes via theme.json. Whereas the alternative is to include all of them within all themes individually.

Start with the pattern creation experience and the StartPageOptionsModal mentioned in the issue, then compare the existing core header and footer workaround in bin/set-core-pattern.php. Review the Twenty Twenty-Four pattern registration and theme.json examples to understand the expected blockType and postType values. Done means authors can set supported block and post types through the directory UI and use the resulting patterns in themes.