resolves #522 upgrade to Asciidoctor.js 2

• upgrade to Asciidoctor.js 2.2.x
• change Asciidoctor.js package import from asciidoctor.js to @asciidoctor/core
• change Opal package import from opal-runtime to asciidoctor-opal-runtime
• allow use of newer patch versions of Asciidoctor.js 2.2.x (via dependency spec ~2.2.0)
• add convert_ prefix to node handler methods in extended converter
• update assertions for Asciidoctor.js behavior
  • max include depth warning
  • nested macro in footnote
  • preserve trailing spaces on non-AsciiDoc include file
  • preserve leading and trailing empty lines in AsciiDoc include file
  • implicit source block when source-language is set
  • docrole renamed to role on Document
• fix printing of stacktrace in CLI
• invert assertion for empty stem block; resolves #663 (closed)