Set up PHPCS #142
Description
Should be fairly straightforward, port over the setup from https://github.com/php-tuf/php-tuf.
I don't see any reason so far that we need to differ in this project.
We should document this as well, though we only have a DEVELOP.md in #132, so we might want to get that PR in first
Separately, we can fix the linting errors (spoiler alert: there are a non-trivial amount of these), and enable linting in CI.