Additional filtering for novelty voices on iOS

[HadrienGardeur]

While testing the demo on an iPad running the latest public beta of 18.4, I noticed that a number of Apple's novelty voices showed up. This seem to be because they're now localized by Apple, which wasn't the case before.

I've handled this issue in my repo by adding the native identifiers for these voices: HadrienGardeur/web-speech-recommended-voices@e42c9f2

In the context of Readium Speech, this will require additional work to handle these identifiers and using them to filter out values based on voiceURI.

[HadrienGardeur]

cc @panaC and @JayPanoz

I'm not sure which one of you will handle that, but it doesn't affect Thorium Desktop. This will be a must have when implementing read aloud in Readium Web though.