feat: template function injection

[whizark]

1. add action hook(s) into the closure that creates a new environment
2. include plain-old-PHP-file at the hook

// A plain-old-PHP-functioin
if (!function_exists('static_function')) {
    function static_function() {
        // do something.
    }
}

// A dynamic function should be injected as a (partial) template variable.
$_dynamic_function = function () {
    // do something.
};

// A template function wrapper.
if (!function_exists('dynamic_function')) {
    function dynamic_function() use ($_dynamic_function) {
        return $_dynamic_function();
    }
}