Added a combinator similar to separated_list0, but that allows for a trailing separator. (doc)tests are mostly complete.

[SleighPenguin22]

The multi module provides a separated_list\[01\]() combinator, but the combinator does not consume a trailing separator, defining my own in my project using a macro is doable, but looks weird when compared to the provided function API.

I'm proposing a combinator whose interface is identical to separated_list[01](sep, parser), but it accepts an optional trailing separator and discards it.

[SleighPenguin22]

Now that I think of it, perhaps the SeparatedList[01] Structs could instead have a boolean field that specifies if a trailing separator should be consumed at runtime?
the constructor then constructs the corresponding struct with the field set to true or false depending on the called constructor (i.e. separated_list0(...) sets it false while separated_list_trailing0(...) sets it true).