Add new methods for base /taxonomies routes & update tests

Because the new /taxonomies route have no $type parameter, and $type
cannot be removed without breaking back-compat, we need to add a new
set of route methods for the new, "correct" /taxonomies routes. To
minimize confusion, the old route method names were left as is, and
new methods added for each method which had previously taken a $type.
All the old methods are marked @deprecated and have a @see reference
to their successors.

New methods:

- `get_taxonomy` now wraps the new `get_taxonomy_object`
- `prepare_taxonomy` now wraps the new `prepare_taxonomy_object`
- `get_terms` now wraps the new `get_taxonomy_terms`
- `get_term` now wraps the new `get_taxonomy_terms`
- `prepare_term` now wraps the new `prepare_taxonomy_term`

The existing unit tests were changed to point to these new route methods.