Add Lexicon support to Kokoro voice backend (pronunciation dictionary)

Similar to https://github.com/karashiiro/TextToTalk/issues/271, I noticed that Lexicons (the files that contain the pronunciation corrections) don't show up for the Kokoro voice backend. I understand the reason why this may have been omitted, because one voice backend, may not need the or maybe even work out of the box with Lexicon files. Since I'm probably one of the only ones both interested and capable of implementing this myself, I'm willing to take on this project, as I'll want to have this set up before I continue my MSQ in-game. But I'll be asking for a code review before I merge anything in to the main branch.

The big design question I've yet to answer is whether each voice backend should have their own separate lexicons or if they can share. First things first, I'm going to try to get them to work with the existing Lexicons, and since that's no extra work.

It might be that it's best to have separate Lexicons, but allow users to enable unsupported lexicons for a particular voice backend.