Refactor: use $_SERVER instead of getenv()

[bpolaszek]

The current implementation does not permit storing the default address to listen to, as it is hardcoded.

By using $_SERVER instead of getenv() to check for the X_LISTEN env var, one could use a dotenv library such as symfony/dotenv to read .env files and populate X_LISTEN.

This could help devs override the default port to listen to (in case something else already use 127.0.0.1:8080 in their projects), and override the default port in their .env.local.

Example here: https://github.com/bpolaszek/freddie/blob/main/.env.test#L11 (this actually doesn't work and I had to specify the "real" env var in the CI)

[clue]

@bpolaszek Thank you for looking into this, changes LGTM!

We've discussed this in our team with the pros and cons of the various ways to access environment variables in PHP and I agree that the $_SERVER variable should be supported out of the box!

I'll use this as a starting point for a future follow-up PR to see how we can also take changes to $_ENV and getenv() into account.

Keep it up! 👍