Always capitalize last word in subtitle

[dstillman]

https://twitter.com/zotero/status/1635408908947795974

We already capitalize prepositions when they're the last word in a title, but we don't currently do the same when they're the last word before a colon. As our tweet notes, the position isn't really the issue, and we can't reliably fix all cases of this programmatically, but at least for cases where there's a preposition before a colon we can get it right.

[zepinglee]

The current spec say:

In both cases, stop words are lowercased, unless they are the first or last word in the string, or follow a colon.

I guess we should add the case of a word before a colon (or em dash).