Add Post Template layout fallback styles for classic themes

[isabel_brison]

Fallback styles need to be added to class-wp-theme-json.php for classic themes to display the correct Post Template layout. This is necessary due to the changes from ​https://github.com/WordPress/gutenberg/pull/49050.

Depends on #58554 for the grid layout styles.

Trac ticket: https://core.trac.wordpress.org/ticket/58570

Adds the Post Template fallback styles for classic themes from ​https://github.com/WordPress/gutenberg/pull/49050, following the same logic already used for the Columns block.

Do ​these unit tests need to be ported over? Or will they be taken care of in #4625?

I think I need to commit #4625 first and then rebase this one, because the tests are still passing without updating the string.

Hmmm, looks like the fallback styles are not being output yet, because the packages haven't been updated, so Post Template block still doesn't have spacing support, so ​this check fails 😞

This is a small PR so I guess it can wait until the packages are updated, and once they are I can update the unit test string.

That approach makes sense to me as well!

[audrasjb]

Converting to a task as this can ship after beta 1 is released.

[isabel_brison]

In 56083:

Editor: add Post Template fallback styles.

Adds fallback styles necessary for classic themes to display the correct Post Template layout.

Props mikeschroder, ramonopoly.
Fixes #58570.

committed in r56083 / ​518a4fc.