Update tokens to target es5

[lavelle]

Right now the ES Module build uses ES6 modules and ES6 syntax in the TypeScript config. We should target ES6 modules but ES5 syntax, to ensure no ES6 code can accidentally slip into IE11 when consumed by clients.

This is not a problem for the thumbprint-react package, since it uses Rollup and babel-plugin-typescript, rather than the tsc command directly.

cc @danoc