Lay the groundwork for a Rust-based eval (#28488)

Also, we moved the logic for driving the agentic loop into `Thread` so
that we don't have to re-implement it.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

Cargo.toml

@@ -47,6 +47,7 @@ members = [
     "crates/diagnostics",
     "crates/docs_preprocessor",
     "crates/editor",
+    "crates/eval",
     "crates/evals",
     "crates/extension",
     "crates/extension_api",