First class callable syntax supported for functions and methods

[spaze]

This is the syntax supported by PHP 8.1:

$func = print_r(...);
$func();

or

$obj = new Object();
$func = $obj->method(...);
$func();

or

$func = Class::method(...);
$func();

The errors for the disallowed code are reported on lines with (...), not when the callable is called.

Ref #275