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/src/slash_command/prompt_command.rs

@@ -1,113 +0,0 @@
-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(crate) 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())
-        })
-    }
-}