Automated taxes: create tax classes for supported tax categories

As discussed with @mikkamp in p6q7sZ-3ZV-p2, TaxJar supports certain product categories and tax exemptions. Their documentation for setup is here: https://support.taxjar.com/article/309-overriding-tax-rates-and-exempting-products-in-woocommerce

Using the [`/v2/categories`](https://developers.taxjar.com/api/reference/#get-list-tax-categories) endpoint, we can retrieve a list of supported tax categories and create tax classes based on them as a convenience for our users.

From Jake@TaxJar:

> We plan to pick up the pace and introduce more categories this year, but in general I'd say not very often. Quarterly moving forward. Our "Release Notes" blog series will announce the new categories and I hope to launch an API changelog soon to keep track of changes.
>
> https://blog.taxjar.com/taxjar-release-notes-november-2017/

Start with the TaxJar /v2/categories endpoint and the linked TaxJar documentation to understand the supported categories and exemptions. Done means the integration retrieves those categories and creates corresponding tax classes as a user convenience.

Written by the indexing model from the issue text.