Provide `original` vocabulary to presave and update vocabulary hooks

## Description of the need

When vocabularies were entities in D7, whenever you updated a vocabulary, the presave and update hooks received the original (unchanged) vocabulary under $entity->original. In Backdrop, when vocabularies were demoted to config classes, this feature was removed.

It would be very helpful to bring back this feature. It's not hard to do.

## Proposed solution

In the `save` method of TaxonomyVocabulary, we'd need to load the original vocabulary before it's saved, and add it to the vocabulary object under `original`. We would also need to create a `vocabulary_load_unchanged()` function to circumvent the cache and load the original vocabulary before it's saved.

I can probably provide a PR for this soon. But I'd like to ask if there is interest in this feature?