Default --config if one is found on disk

[JoshuaKGoldberg]

🚀 Feature Request

It'd be nice if running npx typestat after creating a typestat.json config file didn't require a --config typestat.json. Seems redundant. Extra user work.

Existing Behavior

No auto-detection of a typestat.json is done.

Change Proposal

If a typestat.json exists on disk, default the --config argument to point to it.