Apply a common codestyle + tooling to validate & enforce it

[sanderdlm]

This PR should be reviewed per commit. The first one https://github.com/hollodotme/fast-cgi-client/commit/5da4f532d68830d33d3a1f239922de604be94137 installs the CodeSniffer package and adds some configuration.

The second commit is optional, because it applies the PSR12 codestyle to the entire codebase: https://github.com/hollodotme/fast-cgi-client/commit/88de30c92e6418385a5a32441cdd9c23bdea624d

If you want to go with a different set of rules, we can omit the second commit and apply another one. I chose PSR12 because it is common in the PHP world and seemed like a good standard, but I'm open to others.