Document voice quality as a new property

[HadrienGardeur]

In order to:

• facilitate future work on localizing Apple voices (ie #38)
• and provide an easier way to filter/order by voice quality

I'm introducing a new quality property that can list one or more values per voice.

Android's take on describing quality settings feels pretty good, so I'm re-using the same approach:

veryHigh

Very high, almost human-indistinguishable quality of speech synthesis

high

High, human-like quality of speech synthesis

normal

Normal quality of speech synthesis

low

Low, not human-like quality of speech synthesis

veryLow

Very low, but still intelligible quality of speech synthesis

This will be subjective of course and I'm not necessarily going to follow what's documented by Apple/Google through their native APIs, as I don't really agree with how they rate voice quality.

This property will be optional and I'll skip it in a few cases when it's hard to document it.

[HadrienGardeur]

@Falcosc I think that this was what you were waiting for. I'm busy working on a few things related to comics and making them accessible for the rest of the month, but I'll do my best to continue documenting this property in all languages.