Add CJS -> ESM transpiler

[matthewp]

Add a transpile that will transpile CommonJS to ESM format.

"use strict";

var foo = require("foo");

module.exports = foo;

becomes:

import foo from "foo";

export default foo;

[matthewp]

By default this should only work with strict mode modules. Those that have "use strict"; at the top of the file. If transpile encounters any module that's not strict mode it should throw as it is impossible to guarantee the module will work once converted to ES format.

To work around this, allow an { unsafe: true } option that ignores whether strict mode is there or not.