Extract SlashCommand trait from assistant (#12252)

This PR extracts the `SlashCommand` trait (along with the
`SlashCommandRegistry`) from the `assistant` crate.

This will allow us to register slash commands from extensions without
having to make `extension` depend on `assistant`.

Release Notes:

- N/A

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -0,0 +1,50 @@
+mod slash_command_registry;
+
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+
+use anyhow::Result;
+use futures::channel::oneshot;
+use gpui::{AppContext, Task};
+
+pub use slash_command_registry::*;
+
+pub fn init(cx: &mut AppContext) {
+    SlashCommandRegistry::default_global(cx);
+}
+
+pub trait SlashCommand: 'static + Send + Sync {
+    fn name(&self) -> String;
+    fn description(&self) -> String;
+    fn complete_argument(
+        &self,
+        query: String,
+        cancel: Arc<AtomicBool>,
+        cx: &mut AppContext,
+    ) -> Task<Result<Vec<String>>>;
+    fn requires_argument(&self) -> bool;
+    fn run(&self, argument: Option<&str>, cx: &mut AppContext) -> SlashCommandInvocation;
+}
+
+pub struct SlashCommandInvocation {
+    pub output: Task<Result<String>>,
+    pub invalidated: oneshot::Receiver<()>,
+    pub cleanup: SlashCommandCleanup,
+}
+
+#[derive(Default)]
+pub struct SlashCommandCleanup(Option<Box<dyn FnOnce()>>);
+
+impl SlashCommandCleanup {
+    pub fn new(cleanup: impl FnOnce() + 'static) -> Self {
+        Self(Some(Box::new(cleanup)))
+    }
+}
+
+impl Drop for SlashCommandCleanup {
+    fn drop(&mut self) {
+        if let Some(cleanup) = self.0.take() {
+            cleanup();
+        }
+    }
+}