Add and refactor text statistics functionality

[bdewilde]

Description

• implements a variety of text statistics measuring lexical diversity
  • ttr(): type-token-ratio, in three variations (standard, root, corrected)
  • log_ttr(): logarithmically-scaled TTR, in three variations (herdan, summer, dugast)
  • segmented_ttr(): segmented TTR, in two variations (mean, moving average)
  • mtld(): Measure of Textual Lexical Diversity, a modern and text length-agnostic measure
  • hdd(): Hypergeometric Distribution Diversity, a(nother) modern and text length-agnostic measure
• implements functions for counting morphological, part-of-speech, and dependency annotations in a document
• updates all text statistics functions to accept a Doc object as their first positional argument (i.e. Callable[[Doc, ...], int | float] , so they're more directly usable and accessible
  • adds caching to some functions and improves performance of .get_words() utils func to reduce overhead
• consolidates all TextStats readability properties under a single TextStats.readability() method, where individual statistics are specified by name; similarly, adds TextStats.diversity() and .counts() methods for accessing those statistics
• replaces TextStatsComponent custom spaCy language component with a suite of Doc property and method extensions, settable via textacy.text_stats.set_doc_extensions(); the outcome is roughly the same -- various text stats accessible via spaCy's Doc._ -- but the process is different

Motivation and Context

There are lots of use cases and methods for quantifying aspects of a document, and I wanted textacy to capture more of them. I even used something like annotation counts for a research project. Also, the text_stats functionality was clunky in many ways, and I wanted to make it easier for people to use.

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.