Added pre-commit Action and pre-commit formatting fixes

[wermos]

As requested in #39, this adds a pre-commit GitHub Action for commits to main and PRs. The .pre-commit-config.yaml and the GitHub Action script are taken verbatim from bemanproject/inplace-vector. (Here and here respectively, to be precise.)

A large portion of the diff for this PR is simply the changes I had to make to appease the pre-commit CI. I tried to be as minimally invasive to the README as possible.

This PR, along with #39, addresses issue #3 entirely.

[neatudarius]

@wermos is this branch up to date with main? I was expecting to not change the code files and to have them updated after #39

[wermos]

is this branch up to date with main?

Yes, this branch branches directly off of main. You can verify by checking the parent hash (git log -1 --pretty=%P <commit-hash>) of the first commit in this branch (31d43ad). Its parent is indeed e854abf, which is where main is at right now.

I was expecting to not change the code files and to have them updated after

I'm sorry, I didn't quite get what you mean. Could you clarify that a bit?