## Summary
This PR adds a benchmarking script for the formatter, which benchmarks
the Ruff formatter against Black, yapf, and autopep8.
Three benchmarks are included:
1. Format everything.
2. Format everything, but use a single thread.
3. Format everything, but `--check` (don't write to disk).
There's some nuance in figuring out the right combination of arguments
to each command, but the _main_ nuance is to ensure that we always run
the given formatter (and modify the target repo in-place) prior to
benchmarking it, so that the formatters aren't disadvantaged by the
existing formatting of the target repo. (E.g.: prior to benchmarking
Black's preview style, we need to make sure we format the target repo
with Black's preview style; otherwise, preview style appears much
slower.)
Part of https://github.com/astral-sh/ruff/issues/7309.
