External program "use shell" option can be used to use a new JS context

Rather than introduce yet-another-external-program/config flag to enable
the "new JS run-time/context" per invocation of any particular JS command,
I'm using the XTRN/EX_SH ("Use Shell") option as it had no function for
JavaScript command-lines and the concept is similar.

So if you want to invoke a JS external program/timed-event in its own
"sandbox", enable the "Use Shell / New Context" option for that
program/command in SCFG. There may be performance penalties when running
JS mods in this way, but you get the benefit of isolating one JS module
from all others, if/when necessary.