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

---------

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

crates/assistant_slash_command/Cargo.toml

@@ -15,9 +15,15 @@ path = "src/assistant_slash_command.rs"
 anyhow.workspace = true
 collections.workspace = true
 derive_more.workspace = true
+futures.workspace = true
 gpui.workspace = true
 language.workspace = true
 parking_lot.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 workspace.workspace = true
+
+[dev-dependencies]
+gpui = { workspace = true, features = ["test-support"] }
+pretty_assertions.workspace = true
+workspace = { workspace = true, features = ["test-support"] }