FileSystem: add resolvePath

[JanTvrdik]

• new feature
• BC break: no

This a port of path.resolve from Node.js.

In reality I only need sth like

public static function resolvePath(string $a, string $b): string
{
	return self::isAbsolute($b) ? $b : self::joinPaths($a, $b);
}

so I'm not sure how useful the more general implementation is.