Refactor `everyvoice demo` code to the submodules

See @joanise's comment:

"Is there any code at all that is shared between the styletts2 demo and the fs2+hfgl one? As I scan, it seems like it's completely separate code, and in that case I would think that this should be placed in `styletts2/cli/demo.py` instead of here.

That would also facilitate adding it to the styletts2 CLI as `styletts2 demo`, actually."

_Originally posted by @joanise in https://github.com/EveryVoiceTTS/EveryVoice/pull/804#discussion_r3276408228_