Clarify in the Howto that each story must define the ‘START’ scene

It is likely that authors will spread a large story across multiple
scripts and load them individually, opposed to writing a single,
behemoth script.  The Howto document says that each script must define
the ‘START’ scene but this is no longer true since LNVL.LoadScript()
now accepts scripts that do not define the scene.  Authors must still
define the ‘START’ scene but only need to make sure they do this
within a single story, not within every script like the Howto
incorrectly suggests.  So this patch clarifies the wording in the
Howto regarding that requirement.
Fix the use of a ClampedArray for the ‘Menu.choices’ property

We want to store the choices in a ClampedArray object but we fail to
call the constructor because we do not call the ‘new()’ method.
Fix crash in ‘set-scene’ instruction when looking for Scene objects

We now store the Scene objects in the LNVL.ScriptEnvironment table.
But the instruction still looks for those objects in the global
environment where we previously kept them.  This patch changes the
instruction so that it looks for scenes in the correct environment,
preventing the crash that happens when it fails to find a scene, which
always happens if we look in the wrong environment.