Make the `say` terminal utility and NSSpeechSynthesizer work with Siri voices

In a WWDC20 talk, Apple says Siri voices are not available in AVSpeechSynthesizer.

The talk is called "Create a seamless speech experience in your apps." Apparently this applies to Catalina as well.

Here's the relevant slide:

I started a thread at macrumors regarding similar problems configuring the system voice to speak text.