Themes: Fix parent theme incorrectly not getting resolved

[joemcgill]

The initial commit reverts https://core.trac.wordpress.org/changeset/59885 (83b9080), to show that this is indeed the source of the regression. The cause seems to be that wp_is_block_theme() is getting called before the themes are fully set up. Ensuring the post type support for setting the editor default-mode for pages only fires after themes are fully set up fixes this issue.

Trac ticket: https://core.trac.wordpress.org/ticket/63062

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[joemcgill]

Made a few updates here.

As an aside: given the way themes get loaded in our PHPUnit suite, I'm not sure it's possible to reproduce this bug in a unit test because the tests run too late. Will keep thinking about it, but given the severity of this bug I think getting a fix in quickly is more important than ensuring we have good test coverage in this case.

[joemcgill]

I've updated the logic here to move setting post type support to a standalone callback, as suggested in this comment. @peterwilsoncc any chance you can take another look at this before beta 2?

[joemcgill]

https://core.trac.wordpress.org/changeset/59968