[Minor] export enforce as a standalone module

[ealush]

Description

Export enforce as an individual module, allowing to consume it as a standalone module.
This supports future plans for leaner validation schemes.

The old API still works, and will be maintained for the foreseeable future.

Usage

// before
import passable from 'passable';

passable('form_name', (pass, enforce) => {
    pass('field_name', 'error_string', () => enforce(1).isNumber());
});

// after
import passable, { enforce } from 'passable';

passable('form_name', (pass) => {
    pass('field_name', 'error_string', () => enforce(1).isNumber());
});