Make slash command output streamable (#19632)

This PR adds support for streaming output from slash commands

In this PR we are focused primarily on the interface of the
`SlashCommand` trait to support streaming the output. We will follow up
later with support for extensions and context servers to take advantage
of the streaming nature.

Release Notes:

- N/A

---------

Co-authored-by: David Soria Parra <davidsp@anthropic.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: David <david@anthropic.com>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Will <will@zed.dev>

crates/assistant_slash_command/Cargo.toml

@@ -18,6 +18,7 @@ derive_more.workspace = true
 futures.workspace = true
 gpui.workspace = true
 language.workspace = true
+language_model.workspace = true
 parking_lot.workspace = true
 serde.workspace = true
 serde_json.workspace = true