Extract slash commands to their own crate (#23261)

This PR extracts the slash command definitions out of the `assistant`
crate and into their own `assistant_slash_commands` crate.

Release Notes:

- N/A

crates/assistant_slash_commands/src/streaming_example_command.rs

@@ -0,0 +1,117 @@
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+use std::time::Duration;
+
+use anyhow::Result;
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandContent, SlashCommandEvent,
+    SlashCommandOutputSection, SlashCommandResult,
+};
+use feature_flags::FeatureFlag;
+use futures::channel::mpsc;
+use gpui::{Task, WeakView};
+use language::{BufferSnapshot, LspAdapterDelegate};
+use smol::stream::StreamExt;
+use smol::Timer;
+use ui::prelude::*;
+use workspace::Workspace;
+
+pub struct StreamingExampleSlashCommandFeatureFlag;
+
+impl FeatureFlag for StreamingExampleSlashCommandFeatureFlag {
+    const NAME: &'static str = "streaming-example-slash-command";
+}
+
+pub struct StreamingExampleSlashCommand;
+
+impl SlashCommand for StreamingExampleSlashCommand {
+    fn name(&self) -> String {
+        "streaming-example".into()
+    }
+
+    fn description(&self) -> String {
+        "An example slash command that showcases streaming.".into()
+    }
+
+    fn menu_text(&self) -> String {
+        self.description()
+    }
+
+    fn requires_argument(&self) -> bool {
+        false
+    }
+
+    fn complete_argument(
+        self: Arc<Self>,
+        _arguments: &[String],
+        _cancel: Arc<AtomicBool>,
+        _workspace: Option<WeakView<Workspace>>,
+        _cx: &mut WindowContext,
+    ) -> Task<Result<Vec<ArgumentCompletion>>> {
+        Task::ready(Ok(Vec::new()))
+    }
+
+    fn run(
+        self: Arc<Self>,
+        _arguments: &[String],
+        _context_slash_command_output_sections: &[SlashCommandOutputSection<language::Anchor>],
+        _context_buffer: BufferSnapshot,
+        _workspace: WeakView<Workspace>,
+        _delegate: Option<Arc<dyn LspAdapterDelegate>>,
+        cx: &mut WindowContext,
+    ) -> Task<SlashCommandResult> {
+        let (events_tx, events_rx) = mpsc::unbounded();
+        cx.background_executor()
+            .spawn(async move {
+                events_tx.unbounded_send(Ok(SlashCommandEvent::StartSection {
+                    icon: IconName::FileRust,
+                    label: "Section 1".into(),
+                    metadata: None,
+                }))?;
+                events_tx.unbounded_send(Ok(SlashCommandEvent::Content(
+                    SlashCommandContent::Text {
+                        text: "Hello".into(),
+                        run_commands_in_text: false,
+                    },
+                )))?;
+                events_tx.unbounded_send(Ok(SlashCommandEvent::EndSection))?;
+
+                Timer::after(Duration::from_secs(1)).await;
+
+                events_tx.unbounded_send(Ok(SlashCommandEvent::StartSection {
+                    icon: IconName::FileRust,
+                    label: "Section 2".into(),
+                    metadata: None,
+                }))?;
+                events_tx.unbounded_send(Ok(SlashCommandEvent::Content(
+                    SlashCommandContent::Text {
+                        text: "World".into(),
+                        run_commands_in_text: false,
+                    },
+                )))?;
+                events_tx.unbounded_send(Ok(SlashCommandEvent::EndSection))?;
+
+                for n in 1..=10 {
+                    Timer::after(Duration::from_secs(1)).await;
+
+                    events_tx.unbounded_send(Ok(SlashCommandEvent::StartSection {
+                        icon: IconName::StarFilled,
+                        label: format!("Section {n}").into(),
+                        metadata: None,
+                    }))?;
+                    events_tx.unbounded_send(Ok(SlashCommandEvent::Content(
+                        SlashCommandContent::Text {
+                            text: "lorem ipsum ".repeat(n).trim().into(),
+                            run_commands_in_text: false,
+                        },
+                    )))?;
+                    events_tx.unbounded_send(Ok(SlashCommandEvent::EndSection))?;
+                }
+
+                anyhow::Ok(())
+            })
+            .detach_and_log_err(cx);
+
+        Task::ready(Ok(events_rx.boxed()))
+    }
+}