🚀 Feature: Rename and split between a core library and a markdownlint extension

[JoshuaKGoldberg]

Bug Report Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

This repository has been named sentences-per-line and acted as a Markdownlint custom rule since its inception. But that's kind of a naming issue: one could theoretically want to use the "how many sentences are there in this line?" logic in a non-Markdownlint context.

For example, eslint/markdown#317 discussed making an ESLint or Prettier equivalent. The discussion ended on a Prettier plugin.

Assigning to myself as a refactor: I'd like to retool this project to really be two things:

• General: standalone "what are the sentences in this line?" helper utility
• Markdownlint: contributed rule that uses the utility

Additional Info

Prior art exists with the "is this JSDoc informative?" logic in:

• https://github.com/JoshuaKGoldberg/are-docs-informative: the general helper utility
• https://github.com/gajus/eslint-plugin-jsdoc/blob/e9a98a3c0de9abf6860513b28cf1220891785c5c/docs/rules/informative-docs.md: a lint rule that uses the utility

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