fix: add `@deprecated` tag to `slugs` document property

[angeloashmore]

Types of changes

• [ ] Chore (a non-breaking change which is related to package maintenance)
• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR adds a @deprecated tag to the slugs property on PrismicDocument.

Slugs are a deprecated featured used before the UID field was introduced. The property will continue to be included in the Rest API response, but usage should be discontinued and migrated to the UID field.

For more details on the slugs property and their deprecation decision, see: https://community.prismic.io/t/what-are-slugs/6493

Checklist:

• [ ] My change requires an update to the official documentation.
• [x] All TSDoc comments are up-to-date and new ones have been added where necessary.
• [x] All new and existing tests are passing.

🦤