Warn on unrecognized config keys with a "did you mean" suggestion

[markselby9]

Summary

Closes #3943.

When a pyrefly.toml / [tool.pyrefly] contains a key Pyrefly doesn't recognize, serde's #[serde(flatten)] catch-all silently swept it into extras, and from_file surfaced it only as a single lumped line:

Extra keys found in config: check-unannotated-def, made-up-key

That tells you something is wrong but not what you meant. This PR replaces it with one warning per unrecognized key and a "did you mean" suggestion when a known option is close:

Unknown config key `check-unannotated-def`. Did you mean `check-unannotated-defs`?
Unknown config key `made-up-key`

(As the issue notes, unknown error kinds already fail to parse — this brings unknown config keys closer to that behavior, while still only warning so users can intentionally share a config across tools.)

Details

• Scoped candidates. Top-level keys are matched against the project-wide options plus ConfigBase settings; [[sub-config]] keys are matched against matches + ConfigBase only — so a sub-config typo never suggests a project-only key like project-includes.
• Shared ranking. The suggestion reuses the edit-distance ranking already used for identifier typos in pyrefly_util::suggest. I generalized that module's internals into one private closest<T> and exposed a best_suggestion_str for plain strings; best_suggestion (used by 6 call sites) now delegates to it with unchanged behavior.
• Migration aliases (e.g. snake_case python_version) are accepted by serde, so they never reach extras and are never flagged.

Test plan

• New tests in crates/pyrefly_config/src/config.rs:
  • unknown key with a close match → warns + suggests
  • unknown key with no close match → warns, no suggestion
  • unknown key inside a [[sub-config]] → warns with the sub-config context + suggestion
  • canonical keys and a snake_case migration alias → no warnings
  • test_known_config_keys_cover_serialized_fields: a sync guard so newly-added (serialized) config fields must be added to the candidate lists
• cargo test -p pyrefly_config — 224 passed.
• cargo test -p pyrefly_util — 71 passed (guards the shared helper refactor).
• Suggestion-exercising modules in the main crate unaffected: test::attributes (187), test::typed_dict (161), test::scope (96), test::enums (61), test::calls (47) — all green.
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema — clean.