Create and use list of preferred voices

[ianb]

Frequently (usually?) the browser won't identify any preferred/default voice, and so we'll have to pick one. Right now we pick the alphabetically first voice.

We should choose a voice more intelligently. For instance my alphabetically first one is Agnes, which is not a good voice. Samantha however is a good voice.

To see the available voices:

speechSynthesis.getVoices();
// To see which one is default:
speechSynthesis.getVoices().filter(v => v.default);

My "default" is Alex, which I've never chosen. My system default is Samantha. Note that Samantha has the string "premium" in the voiceURI.

There may be heuristics on Windows and Linux, but we'll have to look more closely to determine them.

Note that internally we should use null as the preference, and then determine the voice dynamically – and if the user sets an explicit preference we respect that.