Adding immediate pausing support

**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.