Updates to Voice list in settings: removing duplicates, other languages

[e28eta]

This might be a partial fix for #149. There are other ideas in that issue that I haven't looked at.

This PR does the following:

• Add all of the voices that exactly match the current language & region
• Add any voices that match the current language, from a different region, that aren't already in the list.
• Filters out voices that are for other languages
• Filters out duplicate entries
• Only show voice "quality" if it's non-Default. Should be more clear what's happening.
• This PR also uses the system default voice by default, and only uses the in-app voice list if the user has specifically chosen & saved a voice in Enchanted.

After:

Before (truncated Picker list, it's 3-4x longer):

Side note: in order to build & test, I had to do something with swift-markdown-ui. I didn't see #154 / #155 , but made a similar change locally.