leader() support
This binds a headline and its page number within a Table of Content. Specification is here: https://www.w3.org/TR/css-gcpm-3/#leaders
Expected output:
Table of Contents
1. Loomings.....................1
2. The Carpet-Bag...............9
3. The Spouter-Inn.............13
Related issue: https://github.com/Mogztter/asciidoctor-pdf.js/issues/126