Adding immediate pausing support

[sthibaul]

Is your feature request related to a problem? Please describe.

From @eeejay :
“SD supports pausing only at a mark or at the end of an utterance, not an immediate pause to speech like any other media.”

The web speech API apparently wants immediate pause support

Describe the solution you'd like

There should be a way to request immediate pausing, which ignores indexing constraints, and can thus pause anywhere.

[sthibaul]

We'd need to refine exactly what is wanted here: when resuming, should we restart exactly where it was stopped in the audio? Or get back at the beginning of the utterance?