Use builtins

[sfarthin]

See #317 first. Will be a much smaller changeset if that PR is merged.

---

Previously this library depended on a global polyfills (@babel/polyfill) to be compatible with older platforms (e.g. older browsers and node). Babel deprecated this strategy and encourages importing from core-js directly. With babel-preset-env preset, we can automatically import the required polyfills with the useBuiltins options.

For example previously we would get this as a babel output...

$ echo "Promise.resolve().finally();" > myTest.js
$ babel myTest.js
"use strict";

Promise.resolve()["finally"]();

Now with the useBuiltins option, our output looks like this:

$ echo "Promise.resolve().finally();" > myTest.js
$ babel myTest.js
"use strict";

require("core-js/modules/es6.promise");

require("core-js/modules/es6.object.to-string");

require("core-js/modules/es7.promise.finally");

Promise.resolve()["finally"]();

Including the polyfills in the library will increase the size, but solves issues with versions of @babel/polyfill and other globals effecting this library. Another option would be to use babel/runtime, but that would require changes to the consumers. See more details on the tradeoffs.