have the ability to put FSE files in a theme subfolder

[pbearne]

Hi All

Looking at the FSE code and thinking how I would apply this to a theme.

It seems to me that we might what to be able to control where the theme.json and block-template folder are stored in a theme.

for 2 reasons

1. control if a theme is using FSE via setting. if we have the setting in a folder and none in the root then until we set the folder as the location FSE will be off
2. provide more than one set of options

This also makes the folder layout cleaner in current theme that what to support both classic and FSE modes

[pbearne]

I had a go at adding a filter
​https://github.com/pbearne/wordpress-develop/tree/%2353353_folder_filter_for_FSE

But I can't find in the core where the folders are added (found in gutenberg plugin _gutenberg_get_template_file() )