Use REQUEST_URI as fallback

[shoorick]

There are some strange and inconvenient things in route handling:

1. Current procedure use $_GET['url'] only. It works with Apache and mod_rewrite but doesn’t work with PHP built-in development server — tested with 7.4 and 8.2 on Debian GNU/Linux 11.
2. Variable self::$url is useless because it assigned but never used.
3. Functions for $url and self::$url are different but it’s possible to use common function set.
4. The else block is unnecessary.

Suggested code manually tested with PHP built-in development server. I’m going to test it with Apache too.