Add slash commands for adding context into the assistant (#12102)

Tasks

* [x] remove old flaps and output when editing a slash command
* [x] the completing a command name that takes args, insert a space to
prepare for typing an arg
* [x] always trigger completions when  typing in a slash command
* [x] don't show line numbers
* [x] implement `prompt` command
* [x] `current-file` command
* [x] state gets corrupted on `duplicate line up` on a slash command
* [x] exclude slash command source from completion request

Next steps:
* show output token count in flap trailer
* add `/project` command that matches project ambient context
* delete ambient context

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant/src/slash_command/prompt_command.rs

@@ -0,0 +1,88 @@
+use super::{SlashCommand, SlashCommandCleanup, SlashCommandInvocation};
+use crate::PromptLibrary;
+use anyhow::{anyhow, Context, Result};
+use futures::channel::oneshot;
+use fuzzy::StringMatchCandidate;
+use gpui::{AppContext, Task};
+use std::sync::{atomic::AtomicBool, Arc};
+
+pub(crate) struct PromptSlashCommand {
+    library: Arc<PromptLibrary>,
+}
+
+impl PromptSlashCommand {
+    pub fn new(library: Arc<PromptLibrary>) -> Self {
+        Self { library }
+    }
+}
+
+impl SlashCommand for PromptSlashCommand {
+    fn name(&self) -> String {
+        "prompt".into()
+    }
+
+    fn description(&self) -> String {
+        "insert a prompt from the library".into()
+    }
+
+    fn requires_argument(&self) -> bool {
+        true
+    }
+
+    fn complete_argument(
+        &self,
+        query: String,
+        cancellation_flag: Arc<AtomicBool>,
+        cx: &mut AppContext,
+    ) -> Task<Result<Vec<String>>> {
+        let library = self.library.clone();
+        let executor = cx.background_executor().clone();
+        cx.background_executor().spawn(async move {
+            let candidates = library
+                .prompts()
+                .into_iter()
+                .enumerate()
+                .map(|(ix, prompt)| StringMatchCandidate::new(ix, prompt.title))
+                .collect::<Vec<_>>();
+            let matches = fuzzy::match_strings(
+                &candidates,
+                &query,
+                false,
+                100,
+                &cancellation_flag,
+                executor,
+            )
+            .await;
+            Ok(matches
+                .into_iter()
+                .map(|mat| candidates[mat.candidate_id].string.clone())
+                .collect())
+        })
+    }
+
+    fn run(&self, title: Option<&str>, cx: &mut AppContext) -> SlashCommandInvocation {
+        let Some(title) = title else {
+            return SlashCommandInvocation {
+                output: Task::ready(Err(anyhow!("missing prompt name"))),
+                invalidated: oneshot::channel().1,
+                cleanup: SlashCommandCleanup::default(),
+            };
+        };
+
+        let library = self.library.clone();
+        let title = title.to_string();
+        let output = cx.background_executor().spawn(async move {
+            let prompt = library
+                .prompts()
+                .into_iter()
+                .find(|prompt| prompt.title == title)
+                .with_context(|| format!("no prompt found with title {:?}", title))?;
+            Ok(prompt.prompt)
+        });
+        SlashCommandInvocation {
+            output,
+            invalidated: oneshot::channel().1,
+            cleanup: SlashCommandCleanup::default(),
+        }
+    }
+}