An experimental external() mode flag that runs JS modules in their own...

An experimental external() mode flag that runs JS modules in their own separate runtime and context - currently does not work (appears to corrupt the shared JS context). Not actually used currently.
The reason I wanted to run a JS module (actually JS doorgame) in its own context was to get useful error messages (filenames and line numbers) when the module throws an exception. Currently, the error is just reported as occuring in xtrn_sec.js, which isn't helpful or correct.