Adds filler response

[erik-dunteman]

OPTIONAL feature add

Pros:

• shaves off about 20% from the critical period latency (end of user speech to start of reply audio playback)
• lightens the mood

Cons:

• not a game-changing latency improvement
• is one more component for users to engage with
• users need to run a modal run src.fillers::Fillers.prefill command to activate this feature

@charlesfrye let's decide if this is worth it before I update docs.

[charlesfrye]

If it's optional, I say we keep it!

[charlesfrye]

Very fun addition, so let's stick with it!