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/language/src/language.rs

@@ -486,6 +486,8 @@ pub struct CodeLabel {
 pub struct LanguageConfig {
     /// Human-readable name of the language.
     pub name: Arc<str>,
+    /// The name of this language for a Markdown code fence block
+    pub code_fence_block_name: Option<Arc<str>>,
     // The name of the grammar in a WASM bundle (experimental).
     pub grammar: Option<Arc<str>>,
     /// The criteria for matching this language to a given file.
@@ -609,6 +611,7 @@ impl Default for LanguageConfig {
     fn default() -> Self {
         Self {
             name: "".into(),
+            code_fence_block_name: None,
             grammar: None,
             matcher: LanguageMatcher::default(),
             brackets: Default::default(),
@@ -1185,6 +1188,13 @@ impl Language {
         self.config.name.clone()
     }
 
+    pub fn code_fence_block_name(&self) -> Arc<str> {
+        self.config
+            .code_fence_block_name
+            .clone()
+            .unwrap_or_else(|| self.config.name.to_lowercase().into())
+    }
+
     pub fn context_provider(&self) -> Option<Arc<dyn ContextProvider>> {
         self.context_provider.clone()
     }