Handle more kinds of overflow triggers for addendum code

[nonprofittechy]

Sometimes when you have a PDF, it has space to list multiple names, but they don't each have a discrete box.

E.g., you can separate the names with a comma, and fit 1, 2, or 3 names, depending on how long they are.

In that scenario, we could trigger an addendum when the length of the list as a string is over a certain length. and we'd want to break the list of names at a full name, not halfway through someone's name.

Addendum doesn't really handle this yet:

1. It doesn't have a concept of calculating the list as a string, it just knows about strings or lists, not strings turned into lists.
2. If something is treated as a string, we don't have a way to cut off the list at an appropriate place. It will just be based on a whole word, but that might be just someone's first name, or even just part of their first name.

[nonprofittechy]

Maybe we need the ability to define the addendum being triggered in a code block, rather than forcing people to use the overflow_trigger method? or maybe a combined approach?

maybe: overflow_calculation_method = "trigger"?

we don't really need to mess around with the "safe value" function, we can make people have their own way to print out what they want.