Added a volume control feature to TTS plugin with analytics

[CharVaibhav]

Hey! I've added a much-needed volume control to the text-to-speech feature. This should help readers who find the default volume either too loud or too quiet.

Changes

1. Added a simple volume button with a slider to the read-aloud toolbar
2. The volume slider appears when clicking the button and hides when clicking elsewhere
3. Created a setVolume method that works with our existing TTS engines
4. Added analytics so we can see how people are using this feature