🎨 Move default theme markup to the `app` layout

[Log1x]

After testing with Debug Bar, it appears that moving the default theme markup to the app layout does not have any negative side-effects like before seen with Sage 9 (e.g. #2116). Another pair of eyes to confirm would be cool.

Alongside being cleaner to maintain this stuff in Blade, this also fixes a lot of inconsistencies/rendering issues with Acorn's exception handler.

Change log

• 🎨 Move default theme markup to the app layout
• 🐛 Fix uncontrollable HTML output (Fixes #3166)
• 🔧 Remove index.php from the Tailwind config
• 🩹 Move get_header hook above wp_head() (Fixes #3171)

[jeremylind]

I think wp_head() needs to be called in index.php outside the app context in order to properly enqueue scripts. In my testing, with a block with block.json defining a view script, the script is not enqueued when wp_head() is in the app layout.