assistant: Update `SlashCommand` trait with streaming return type (#19652)

This PR updates the `SlashCommand` trait to use a streaming return type.

This change is just at the trait layer. The goal here is to decouple
changing the trait's API while preserving behavior on either side.

The `SlashCommandOutput` type now has two methods for converting two and
from a stream to use in cases where we're not yet doing streaming.

On the `SlashCommand` implementer side, the implements can call
`to_event_stream` to produce a stream of events based off the
`SlashCommandOutput`.

On the slash command consumer side we use
`SlashCommandOutput::from_event_stream` to convert a stream of events
back into a `SlashCommandOutput`.

The `/file` slash command has been updated to emit `SlashCommandEvent`s
directly in order for it to work properly.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>