Support AsciiDoc.py (AsciiDoctor compat-mode)

AsciiDoctor [changed](https://docs.asciidoctor.org/asciidoctor/latest/migrate/asciidoc-py/) some aspects of formatting from AsciiDoc.py with meaningful consequences for syntax highlighting.

- `'italics'` by single-quoting
- `` `LaTeX-style' `` curly single quotes
- ``` ``LaTeX-style'' ``` curly double quotes
