[PATCH] add commonjs require support for enforce

[ealush]

Change details

#58 introduced an issue in which using commonjs requires had to require('passable').default due to multiple exports in the same module.

By assigning enforce directly to passable (passable.enforce), we're maintaining both

const passable = require('passable');
const enforce = passable.enforce;

and

import passable, {enforce} from 'passable';