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/prompt_command.rs

@@ -0,0 +1,113 @@
+use anyhow::{anyhow, Context, Result};
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
+};
+use gpui::{Task, WeakView};
+use language::{BufferSnapshot, LspAdapterDelegate};
+use prompt_library::PromptStore;
+use std::sync::{atomic::AtomicBool, Arc};
+use ui::prelude::*;
+use workspace::Workspace;
+
+pub struct PromptSlashCommand;
+
+impl SlashCommand for PromptSlashCommand {
+    fn name(&self) -> String {
+        "prompt".into()
+    }
+
+    fn description(&self) -> String {
+        "Insert prompt from library".into()
+    }
+
+    fn icon(&self) -> IconName {
+        IconName::Library
+    }
+
+    fn menu_text(&self) -> String {
+        self.description()
+    }
+
+    fn requires_argument(&self) -> bool {
+        true
+    }
+
+    fn complete_argument(
+        self: Arc<Self>,
+        arguments: &[String],
+        _cancellation_flag: Arc<AtomicBool>,
+        _workspace: Option<WeakView<Workspace>>,
+        cx: &mut WindowContext,
+    ) -> Task<Result<Vec<ArgumentCompletion>>> {
+        let store = PromptStore::global(cx);
+        let query = arguments.to_owned().join(" ");
+        cx.background_executor().spawn(async move {
+            let prompts = store.await?.search(query).await;
+            Ok(prompts
+                .into_iter()
+                .filter_map(|prompt| {
+                    let prompt_title = prompt.title?.to_string();
+                    Some(ArgumentCompletion {
+                        label: prompt_title.clone().into(),
+                        new_text: prompt_title,
+                        after_completion: true.into(),
+                        replace_previous_arguments: true,
+                    })
+                })
+                .collect())
+        })
+    }
+
+    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 title = arguments.to_owned().join(" ");
+        if title.trim().is_empty() {
+            return Task::ready(Err(anyhow!("missing prompt name")));
+        };
+
+        let store = PromptStore::global(cx);
+        let title = SharedString::from(title.clone());
+        let prompt = cx.background_executor().spawn({
+            let title = title.clone();
+            async move {
+                let store = store.await?;
+                let prompt_id = store
+                    .id_for_title(&title)
+                    .with_context(|| format!("no prompt found with title {:?}", title))?;
+                let body = store.load(prompt_id).await?;
+                anyhow::Ok(body)
+            }
+        });
+        cx.foreground_executor().spawn(async move {
+            let mut prompt = prompt.await?;
+
+            if prompt.starts_with('/') {
+                // Prevent an edge case where the inserted prompt starts with a slash command (that leads to funky rendering).
+                prompt.insert(0, '\n');
+            }
+            if prompt.is_empty() {
+                prompt.push('\n');
+            }
+            let range = 0..prompt.len();
+            Ok(SlashCommandOutput {
+                text: prompt,
+                sections: vec![SlashCommandOutputSection {
+                    range,
+                    icon: IconName::Library,
+                    label: title,
+                    metadata: None,
+                }],
+                run_commands_in_text: true,
+            }
+            .to_event_stream())
+        })
+    }
+}