[Feature request] Permit control of speech rate and add (pause) to nonverbals

Currently, the only way to control the apparent speech rate is via the Speed Factor control, which, unfortunately—but understandably—also shifts the pitch of the audio. Ideally, we should be able to have more explicit control over the pronunciation rate of the words for different speech use cases without pitch-shifting the audio.

Additionally, having a (pause) nonverbal to inject a silent nonverbal would also help control the rate of speech. Optimally, a (pause) nonverbal should be fixed-length, short, and stackable to enable more precise control of speech patterns.