deps: import acorn@5.2.1

Imported from the tarball published on npm
(https://registry.npmjs.org/acorn/-/acorn-5.2.1.tgz).

PR-URL: https://github.com/nodejs/node/pull/15566


Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>