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/search/src/project_search.rs

@@ -1067,7 +1067,7 @@ impl ProjectSearchView {
             let range_to_select = match_ranges[new_index].clone();
             self.results_editor.update(cx, |editor, cx| {
                 let range_to_select = editor.range_for_match(&range_to_select);
-                editor.unfold_ranges([range_to_select.clone()], false, true, cx);
+                editor.unfold_ranges(&[range_to_select.clone()], false, true, cx);
                 editor.change_selections(Some(Autoscroll::fit()), cx, |s| {
                     s.select_ranges([range_to_select])
                 });