Why:
1) users can install .scm scripts to plug-ins dir
2) Crashing scripts do not crash extension-script-fu
Scripts (.scm files) have a shebang and are executable
and in a same-named subdir of plugin dir.
Interpreter/scripts create PDB procs of type PLUGIN unlike extension-script-fu
which creates PDB procs of type TEMPORARY, owned by extension-script-fu.
Unlike other interpreters, the interpreter is-a plugin outright,
not by virtue of the script subclassing GimpPlugin and using GI.
More details in /plug-ins/script-fu/interpreter/README
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