ScriptFu: fix #12208 enhance loading of init and compat scripts

Move renamed init and compat scripts to a subdirectory scripts/scriptfu-init

ScriptFu not automatically load all scripts in the directory, only a subset.

Scripts can optionally load compat scripts from that well-known directory, known as script-fu-sys-init-directory.

Users can keep similar scripts in their configuration directory, known as script-fu-user-init-directory. Similarly only loaded on demand using the load function.

No intended changes to behavior of the app or scripts in the repo.

Publish partial compatibility scripts for older GIMP 2 scripts. Loadable at run-time by scripts. This is the part that eases porting.