Mdast extension to handle nodes inserted by micromark-extension-attention.

• Create branch
• Create .remarkignore and add notes.md
• ./index.js: rename to attentionFrom/ToMarkdown
• ./lib/index.js: parameterize attentionTo/FromMarkdown, move handleStrikethrough and handleStrikethrough.peek inside attentionToMarkdown to parameterize
• Rename test.js .test.js
• Run task npm prepack
• Push

Note that the shipped npm prepack overwrites custom annotations in the .d.ts files and the eslint errors on import types. Also an .eslintrc had to be added to fix spurious errors in the source.