Add support for Asciidoc

[danvk]

Is your feature request related to a problem? Please describe.

I'm writing a book and the publisher requires that I use Asciidoc. I'd like to use this extension to check my grammar.

Describe the solution you'd like

I'd like Asciidoc to be a supported markup language.

Describe alternatives you've considered

My workaround is to change VS Code's language mode from Asciidoc → Markdown. This enables vscode-ltex, but Asciidoc and Markdown aren't the same, so this also has some downsides.

Additional context

This came up in #277 (comment) but I don't believe an issue was ever filed for Asciidoc specifically.

[Kasi07]

I'd love this feature too.
As I understand from this comment a java parser is necessary. There are official Java bindings for Asciidoctor called AsciidoctorJ. Maybe these could make this possible?