Implement ExcerptList::subscribe

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/language/src/buffer.rs

@@ -28,6 +28,7 @@ use std::{
     time::{Duration, Instant, SystemTime, UNIX_EPOCH},
     vec,
 };
+use text::subscription::Subscription;
 pub use text::{Buffer as TextBuffer, Operation as _, *};
 use theme::SyntaxTheme;
 use tree_sitter::{InputEdit, Parser, QueryCursor, Tree};