Add Buffer::language_at, update MultiBuffer to use it

Co-authored-by: Julia Risley <floc@unpromptedtirade.com>

crates/language/src/tests.rs

@@ -1449,7 +1449,7 @@ fn get_tree_sexp(buffer: &ModelHandle<Buffer>, cx: &gpui::TestAppContext) -> Str
     buffer.read_with(cx, |buffer, _| {
         let snapshot = buffer.snapshot();
         let layers = snapshot.syntax.layers(buffer.as_text_snapshot());
-        layers[0].2.to_sexp()
+        layers[0].node.to_sexp()
     })
 }