Make sure Humanize(LetterCasing.Sentence) does not remove commas

[louis-z]

• Adapt code so that mid-sentence punctuation is kept when humanizing to a sentence
• Split a regex into simpler constituent parts, easier to reason about
• Add tests

Fixes #958

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[SimonCropp]

thanks for the contribution

[louis-z]

thanks for the contribution

Sure, no problem. I just rebased my branch. It was rather stale... 😆

[SimonCropp]

@louis-z thanks. merged