Allow header.php and footer.php fallback if header.html and footer.html parts are not used

[dansdev]

This was brought up a few years back and there doesn't seem to be anything in place yet - #33989

Currently in templates if you have a .php file with a higher specificity than your .html file, the .php file will take priority in rendering. E.g - if you have singular.html in your /block-templates/ dir, and a single.php in your normal directory, the single.php will take priority over the singular.html.

Integrating this same logic for header.php and footer.php files would be amazing!

[carolinan]

Duplicate of #33989