File context for assistant panel (#9712)

Introducing the Active File Context portion of #9705. When someone is in
the assistant panel it now includes the active file as a system message
on send while showing them a nice little display in the lower right:


![image](https://github.com/zed-industries/zed/assets/836375/9abc56e0-e8f2-45ee-9e7e-b83b28b483ea)

For this iteration, I'd love to see the following before we land this:

* [x] Toggle-able context - user should be able to disable sending this
context
* [x] Show nothing if there is no context coming in
* [x] Update token count as we change items
* [x] Listen for a more finely scoped event for when the active item
changes
* [x] Create a global for pulling a file icon based on a path. Zed's
main way to do this is nested within project panel's `FileAssociation`s.
* [x] Get the code fence name for a Language for the system prompt
* [x] Update the token count when the buffer content changes

I'm seeing this PR as the foundation for providing other kinds of
context -- diagnostic summaries, failing tests, additional files, etc.

Release Notes:

- Added file context to assistant chat panel
([#9705](https://github.com/zed-industries/zed/issues/9705)).

<img width="1558" alt="image"
src="https://github.com/zed-industries/zed/assets/836375/86eb7e50-3e28-4754-9c3f-895be588616d">

---------

Co-authored-by: Conrad Irwin <conrad@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/assistant/src/embedded_scope.rs

@@ -0,0 +1,91 @@
+use editor::MultiBuffer;
+use gpui::{AppContext, Model, ModelContext, Subscription};
+
+use crate::{assistant_panel::Conversation, LanguageModelRequestMessage, Role};
+
+#[derive(Default)]
+pub struct EmbeddedScope {
+    active_buffer: Option<Model<MultiBuffer>>,
+    active_buffer_enabled: bool,
+    active_buffer_subscription: Option<Subscription>,
+}
+
+impl EmbeddedScope {
+    pub fn new() -> Self {
+        Self {
+            active_buffer: None,
+            active_buffer_enabled: true,
+            active_buffer_subscription: None,
+        }
+    }
+
+    pub fn set_active_buffer(
+        &mut self,
+        buffer: Option<Model<MultiBuffer>>,
+        cx: &mut ModelContext<Conversation>,
+    ) {
+        self.active_buffer_subscription.take();
+
+        if let Some(active_buffer) = buffer.clone() {
+            self.active_buffer_subscription =
+                Some(cx.subscribe(&active_buffer, |conversation, _, e, cx| {
+                    if let multi_buffer::Event::Edited { .. } = e {
+                        conversation.count_remaining_tokens(cx)
+                    }
+                }));
+        }
+
+        self.active_buffer = buffer;
+    }
+
+    pub fn active_buffer(&self) -> Option<&Model<MultiBuffer>> {
+        self.active_buffer.as_ref()
+    }
+
+    pub fn active_buffer_enabled(&self) -> bool {
+        self.active_buffer_enabled
+    }
+
+    pub fn set_active_buffer_enabled(&mut self, enabled: bool) {
+        self.active_buffer_enabled = enabled;
+    }
+
+    /// Provide a message for the language model based on the active buffer.
+    pub fn message(&self, cx: &AppContext) -> Option<LanguageModelRequestMessage> {
+        if !self.active_buffer_enabled {
+            return None;
+        }
+
+        let active_buffer = self.active_buffer.as_ref()?;
+        let buffer = active_buffer.read(cx);
+
+        if let Some(singleton) = buffer.as_singleton() {
+            let singleton = singleton.read(cx);
+
+            let filename = singleton
+                .file()
+                .map(|file| file.path().to_string_lossy())
+                .unwrap_or("Untitled".into());
+
+            let text = singleton.text();
+
+            let language = singleton
+                .language()
+                .map(|l| {
+                    let name = l.code_fence_block_name();
+                    name.to_string()
+                })
+                .unwrap_or_default();
+
+            let markdown =
+                format!("User's active file `{filename}`:\n\n```{language}\n{text}```\n\n");
+
+            return Some(LanguageModelRequestMessage {
+                role: Role::System,
+                content: markdown,
+            });
+        }
+
+        None
+    }
+}