Add --pdf-outlines option(s) to assign PDF outlines based on pages and Markdown headings

[yhatt]

Resolves #478.

Structure

1. (Marpit plugin) After parsing Markdown, mark markdown-it's heading tokens with unique key, to make recognizable heading items in browser.
2. (Puppeteer) Before converting Markdown to PDF, the browser finds marked heading items and resolves the rendered positions and displayed texts: offsetLeft, offsetTop, and textContent.
3. (pdf-lib) Postprocess the output PDF to assign document outlines, from the result of 1 and 2.

setOutline() in utils/pdf.ts supports assigning outlines/bookmarks to PDF, for jumping into the page or the specific position of the page.

ToDo

• Assign PDF outlines
• CLI options (--pdf-outlines)
  • Consider about detailed options (e.g. --pdf-outlines.pages, --pdf-outlines.headings)
• Tests
• Update README.md