Motivation
Now that we have codecov, I can see the slow tests:
https://app.codecov.io/gh/nathanjmcdougall/usethis-python/tests/main?parameter=SLOWEST_TESTS

Let's work to make them faster (e.g. through #284 and #285). To help with that, let's benchmark them with codspeed (related to #209; but differs since that seeks to cover the CLI surface whereas this seeks to cover the slow tests).

Summary of desired enhancement
Add the benchmark decorator to the top ~30 slowest functions so we don't make them any slower (and hopefully make them faster).