Add Buffer::outline method

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

Cargo.lock

@@ -3121,6 +3121,17 @@ version = "2.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "afb2e1c3ee07430c2cf76151675e583e0f19985fa6efae47d6848a3e2c824f85"
 
+[[package]]
+name = "outline"
+version = "0.1.0"
+dependencies = [
+ "editor",
+ "gpui",
+ "postage",
+ "text",
+ "workspace",
+]
+
 [[package]]
 name = "p256"
 version = "0.9.0"
@@ -5724,6 +5735,7 @@ dependencies = [
  "log-panics",
  "lsp",
  "num_cpus",
+ "outline",
  "parking_lot",
  "postage",
  "project",