Start work on autocomplete for chat mentions

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Marshall <marshall@zed.dev>

crates/language/src/language.rs

@@ -380,7 +380,9 @@ pub trait LspAdapter: 'static + Send + Sync {
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct CodeLabel {
     pub text: String,
+    /// Determines the syntax highlighting for the label
     pub runs: Vec<(Range<usize>, HighlightId)>,
+    /// Which part of the label participates
     pub filter_range: Range<usize>,
 }