Generalize the block registration and script queuing

Instead of having a `index.php` file in each block and copy it into a builded php file, we should create manually one registration file and one for script queuing. This would decouple the block setup from the compiling of the blocks.

Start by locating the per-block index.php files and the build step that copies them into compiled PHP files. Trace how block registration and script queuing currently happen, then define the manual registration and queuing files needed to decouple setup from compilation. Done means block setup no longer depends on copied per-block index.php files.

Written by the indexing model from the issue text.