Add Buffer::outline method

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/language/src/outline.rs

@@ -0,0 +1,12 @@
+use std::ops::Range;
+
+#[derive(Debug)]
+pub struct Outline(pub Vec<OutlineItem>);
+
+#[derive(Debug)]
+pub struct OutlineItem {
+    pub id: usize,
+    pub range: Range<usize>,
+    pub text: String,
+    pub name_range_in_text: Range<usize>,
+}