🐛 Fix bug in von-part name parsing

[mlutze]

The current system has a name parsing issue around capitalization in comma-style names. This means that

aa bb, CC

gets parsed as

last: aa bb
first: CC
von: <empty>

but BibTeX parses this as

last: bb
first: CC
von: aa

I have modified the parsing code and added relevant test cases drawn from the Tame The BeaST BibTeX reference document.

Style-wise, I found the list manipulation difficult to follow; I added a helper function rindex, which makes it easier to understand IMO. This should probably go in a utility file or something, but I didn't see one in the project.

I believe these changes will fix at least some of the failing test cases in the name merging PR here: #422

[MiWeiss]

Looks great, thanks!