Additional filters on `voices()`

[HadrienGardeur]

In addition to the default filter applies by voices() (getting rid of novelty and very low quality voices), we would like to implementing the following optional filters as well:

• quality, where the input is the minimum value required to meet this criteria
• recommended voices, which would only keep recommended voices and drop everything else
• languages (with optional subtags for regions)
• gender
• and offline availability

I expect the languages filter to be the most useful one by far, as implementers can be tempted to simply filter the output for the voice that they need.