feat(caddy): php_server simplified directive

[dunglas]

Introduce a new simplified php_server directive, which is very similar to the php_fastcgi directive provided by Caddy.

This is now all you need to start a production-grade PHP server:

{
    frankenphp
    order php_server before reverse_proxy
}

localhost {
    encode zstd gzip
    php_server
    file_server
}

Would you mind checking if the code looks good to you and if it follows Caddy's best practices @francislavoie @mholt?

Replaces #92.

[mholt]

How did you know this is just what I always wanted for PHP 😭 and is precisely the reason I didn't originally name the php_fastcgi directive php or php_server -- I knew there had to be a better way to do PHP in the future! 💯 👏

[mholt]

Thank you Francis, for being more thorough 😅 (It's been a stressful day here.)