Clean up get_chapters code and expand tests

[isaac091]

@Enkidu93 For the first section, I thought my original error messages didn't make a ton of sense, so I rewrote and combined them. I also updated the BOOK_RANGE and CHAPTER_SELECTION regexes to allow subtraction so that I could take out the subtraction check right before.

I also decided to just take out the ability to add "chapter 0" because I don't think anyone's using it and it's inconsistent since adding the whole book doesn't include chapter 0, like we talked about.

I kept whole books being represented as an empty list because it's much more convenient that way for silnlp. Otherwise, we'd have to pass a versification variable through all the different functions just to be able to check chapter counts.

Other than that, I fixed the stuff for backwards ranges and added your test cases. Let me know if you have more thoughts or if I missed something!

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This change is 

[ddaspit]

Paratext does have the concept of chapter 0. It includes everything before the first chapter marker. Scripture Forge doesn't have the same concept. Chapter 1 includes all of the introductory material. I guess we should be consistent with Scripture Forge, so it should be okay to remove support for chapter 0.

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[Enkidu93]

Excellent! Thank you, Isaac, for the quick turnaround. I'll make sure the C# matches now after your additional changes and add matching tests.

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