This PR makes queueing a prompt the default behavior when sending a new
one while there's an on-going generation. You can still send a prompt
that will immediately interrupt the agent with the `cmd-shift-enter`
keybinding, though, which preserves the current behavior.

The main motivation for this change is to make the queueing not only
more discoverable, but more useful as well, as we're parting from the
assumption that most of the time, what you want is to queue it as
opposed to interrupting it (even though it's still possible to do either
through the keybinding I mentioned above or simply by stopping the
generation and sending a new one).

Here's a quick video:


https://github.com/user-attachments/assets/37f92433-70ef-459f-98ff-41ed80e3e43f

In the video, I show sending one prompt and then sending two others that
fall straight into the queue. Then, in the middle of the generation of
my first prompt, I use the `cmd-shift-enter` keybinding to send a prompt
immediately, interrupting the agent, effectively being sent in front of
the queue.

Release Notes:

- Agent: Made queueing prompts the default behavior when sending them
while there's an on-going generation.