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/src/slash_command.rs

@@ -127,7 +127,6 @@ impl SlashCommandCompletionProvider {
                                                             &command_name,
                                                             &[],
                                                             true,
-                                                            false,
                                                             workspace.clone(),
                                                             cx,
                                                         );
@@ -212,7 +211,6 @@ impl SlashCommandCompletionProvider {
                                                             &command_name,
                                                             &completed_arguments,
                                                             true,
-                                                            false,
                                                             workspace.clone(),
                                                             cx,
                                                         );