Allow .uv.toml

Like for most of the tools out there, ruff included, would it be possible to allow the configuration file to be prefixed with `.`: `.uv.toml`?

Start by locating uv's configuration-file discovery entry point and the existing handling for uv.toml. Check how configuration precedence and path discovery are tested, then verify that .uv.toml is accepted without changing existing configuration behavior.

Written by the indexing model from the issue text.