#2 Add extended script functions

[Grant McDorman]

Extended functions equivalent to the standard IsScriptRunning and StartScript should be added, i.e.
xIsScriptRunning <string>
and
xStartScript <string>
An additional function, xScriptExists (or a similar name) should be included.

This would allow dynamically detecting other mods and invoking scripts in them.

[Grant McDorman]

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Investigation reveals that in order to find a script by name, the entire list of scripts must be searched.

Thus, it would be better for an 'xGetScriptRef' function (or similar) to be provided, and for xIsScriptRunning and xStartScript to take a reference to a script.