ScriptFu: Add script-fu-interpreter akin to other interpreters

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

plug-ins/script-fu/libscriptfu/script-fu-script.h

@@ -46,5 +46,11 @@ gchar    * script_fu_script_get_command             (SFScript             *scrip
 gchar    * script_fu_script_get_command_from_params (SFScript             *script,
                                                      const GimpValueArray *args);
 
+GimpProcedure * script_fu_script_create_PDB_procedure (GimpPlugIn         *plug_in,
+                                                       SFScript           *script,
+                                                       GimpRunFunc         run_func,
+                                                       GimpPDBProcType     plug_in_type);
+
+
 
 #endif /*  __SCRIPT_FU_SCRIPT__  */