Only show the missing trigger variable warning for Story Table Steps

[plocket]

The trigger variable only relevant for Story Table steps trying to identify unique pages for fields that use index variables or generic objects (see #256). In those cases, the developer needs to add some special lines in their interview to give us that clue. We've written a warning for whenever the developer leaves the trigger variable out of their interview, but it shows up all the time, not just when it would matter.

Ideas for when the warning of a missing trigger variable should be added to the report:

1. Whenever there's a Story Table Step.
2. Whenever a Story Table has any rows that use index variables or generic objects.
3. Whenever a Story Table row uses an index variable or generic object.
4. Whenever a Story Table row includes something in its "trigger" column.
5. Whenever a trigger variable is present in the page. Rejected because a trigger variable would be present on every page of an interview, especially if Assembly Line is included, not just those that really need them.
6. A combination of the above.

Regardless, it shouldn't show up for other types of Steps, even if they set variables. The page where the value is meant to be set is already clear in that situation.

I'm reluctant to pass another parameter into scope.setFields(), but that may be the only way to do it.

Edit: See docs for trigger var: https://suffolklitlab.org/docassemble-AssemblyLine-documentation/docs/automated_integrated_testing/#a-missing-trigger-variable

[plocket]

We might do this by giving non-story-table Steps a fake 'trigger' variable. Something like 'ALKiln: no trigger variable needed'. That way, it won't be undefined. [We'd then check the value of the story table's trigger variable to somehow prevent page field detection from logging that error? I'm unsure. Maybe we move that error up out of page field detection into setFields itself.]

It'd still be nice to reduce this to only printing once per page. [Maybe that could be a separate issue, but taking care of it here would be acceptable.]