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/default_command.rs

@@ -0,0 +1,89 @@
+use anyhow::{anyhow, Result};
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
+};
+use gpui::{Task, WeakView};
+use language::{BufferSnapshot, LspAdapterDelegate};
+use prompt_library::PromptStore;
+use std::{
+    fmt::Write,
+    sync::{atomic::AtomicBool, Arc},
+};
+use ui::prelude::*;
+use workspace::Workspace;
+
+pub struct DefaultSlashCommand;
+
+impl SlashCommand for DefaultSlashCommand {
+    fn name(&self) -> String {
+        "default".into()
+    }
+
+    fn description(&self) -> String {
+        "insert default prompt".into()
+    }
+
+    fn menu_text(&self) -> String {
+        "Insert Default Prompt".into()
+    }
+
+    fn requires_argument(&self) -> bool {
+        false
+    }
+
+    fn complete_argument(
+        self: Arc<Self>,
+        _arguments: &[String],
+        _cancellation_flag: Arc<AtomicBool>,
+        _workspace: Option<WeakView<Workspace>>,
+        _cx: &mut WindowContext,
+    ) -> Task<Result<Vec<ArgumentCompletion>>> {
+        Task::ready(Err(anyhow!("this command does not require argument")))
+    }
+
+    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 store = PromptStore::global(cx);
+        cx.background_executor().spawn(async move {
+            let store = store.await?;
+            let prompts = store.default_prompt_metadata();
+
+            let mut text = String::new();
+            text.push('\n');
+            for prompt in prompts {
+                if let Some(title) = prompt.title {
+                    writeln!(text, "/prompt {}", title).unwrap();
+                }
+            }
+            text.pop();
+
+            if text.is_empty() {
+                text.push('\n');
+            }
+
+            if !text.ends_with('\n') {
+                text.push('\n');
+            }
+
+            Ok(SlashCommandOutput {
+                sections: vec![SlashCommandOutputSection {
+                    range: 0..text.len(),
+                    icon: IconName::Library,
+                    label: "Default".into(),
+                    metadata: None,
+                }],
+                text,
+                run_commands_in_text: true,
+            }
+            .to_event_stream())
+        })
+    }
+}