Extract outline rendering to outline crate (#20179)

This PR extracts the `render_item` implementation for outlines to the
`outline` crate to help reduce `language`'s dependence on `theme`.

Release Notes:

- N/A

Cargo.lock

@@ -7795,6 +7795,7 @@ dependencies = [
  "project",
  "rope",
  "serde_json",
+ "settings",
  "smol",
  "theme",
  "tree-sitter-rust",
@@ -7819,6 +7820,7 @@ dependencies = [
  "language",
  "log",
  "menu",
+ "outline",
  "pretty_assertions",
  "project",
  "schemars",