def global_variable_exists(name)
      assert_type name, :String, :name
      bool(environment.global_env.var(name.value))
    end
def global_variable_exists(name)
      assert_type name, :String, :name
      bool(environment.global_env.var(name.value))
    end
Check whether a variable with the given name exists in the global
 scope (at the top level of the file).

 @example
   $a-false-value: false;
   global-variable-exists(a-false-value) => true
   global-variable-exists(a-null-value) => true

   .foo {
     $some-var: false;
     @if global-variable-exists(some-var) { /* false, doesn't run */ }
   }

 @overload global_variable_exists($name)
   @param $name [Sass::Script::Value::String] The name of the variable to
     check. The name should not include the `$`.
 @return [Sass::Script::Value::Bool] Whether the variable is defined in
   the global scope.