[Post Date] Block controls don't show custom date formats

[kjellr]

It's possible to use the block markup to customize the date format for the post date block. For example:

<!-- wp:post-date {"format":"M","fontSize":"small"} /-->

^ This will display only the month. Formats like this are valid markup, and are interpreted just fine by the editor, except they're not correctly shown in the date format dropdown. If you use a custom format here, the field will just have the default selected:

Instead, if would be great if that could say "Custom" or something like that.

Related: #34762, #35558

[torounit]

It appears to have been fixed in #39209.