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

• Speech commands for synthesizers and speech commands for internal speech processing are not well separated.
• The SpeechManager class is the boundary between internal speech processing and the synthesizer, however it isn't entirely clear which commands may exist on either side of that boundary.
• SynthDrivers have a list of supported commands, however these aren't actually considered anywhere.

Describe the solution you'd like

• Make a new base class for SynthCommands. Commands like PitchCommand should inherit from there.
• Either through design (better), or documentation (worse) make the separation of intents for commands for synth and internal speech commands clear .
• Enforce SynthDriver.supportedCommands from speech manager (with reasonable defaults if the driver provides and empty list).

Reasonable defaults is probably:

	supportedCommands = {
		IndexCommand,
		CharacterModeCommand,
		LangChangeCommand,
		BreakCommand,
		PitchCommand,
		RateCommand,
		VolumeCommand,
		PhonemeCommand,
	}

Describe alternatives you've considered

None

Additional context

While working on #12710 the conflated purposes of speech commands became clear.