Standardize and refactor doc extensions

[bdewilde]

Description

• consolidates all spaCy Doc extension-handling in spacier.extensions module, which includes functionality for "registering" functions that return sets of doc extensions using explosion's catalogue pkg
• refactors doc extension handling in extract and text_stats subpkgs to leverage the new setup
  • a given set of extensions are now set (or removed) by importing the relevant subpkg then calling textacy.set_doc_extensions(NAME), where NAME is a string like "extract" or, optionally, a specific subset like "extract.basics" which registers the corresponding subset of extensions
• refactors pkg-level extensions module
  • general functionality moved to spacier.extensions, core extensions (Doc._.preview and Doc._.meta) moved to spacier.core, and a couple of bag-oriented functions moved to extract.bags, with corresponding extensions settable via textacy.set_doc_extensions("extract") or textacy.set_doc_extensions("extract.bags")
  • removes to_tokenized_text() function + method extension, which was a bit of an odd-ball and was originally meant for interoperability w/ gensim

Motivation and Context

This aspect of textacy was inconsistent and awkward. It had been developed in pieces over multiple major versions of spacy, and never really reconsidered. The current setup is more explicit and configurable from a user's perspective, rather than implicit / one-size-fits-all.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation, and I have updated it accordingly.