Add preview in the style of GitHub

[Fred-Vatin]

VS Code version	OS	Extension version
1.83.0	Win 10 x64 pro 22H2	v3.1.5

I use this extension to help me to create my readme.adoc file and other. I need to see as they would render later on GitHub.

I suggest a setting named asciidoc.preview.defaultStyle presenting a combo menu with these options:

• VS Code (default)
• Asciidoctor
• GitHub (Dark)
• GitHub (Light)

It would replace the current setting asciidoc.preview.useEditorStyle

Or at least, in asciidoc.preview.style provide an URL to a style we could copy/paste to render as GitHub. (Currently external files can not be loaded because of this VS Code bug #187697 already mentioned in #747.)

In the meantime, I'm using this style which you could take inspiration from to integrate it into the addon.

[ggrossetie]

I don't mind including a set of well-know platform styles such as npmjs, GitHub, GitLab...
However, it's important to note that GitHub is not using the same HTML converter. In other words, someone will have to maintain a stylesheet that mimics GitHub style but specially designed for the built-in Asciidoctor HTML converter.

If you are willing to do so, feel free to open a pull request.

I suggest a setting named asciidoc.preview.defaultStyle presenting a combo menu with these options:

• VS Code (default)
• Asciidoctor
• GitHub (Dark)
• GitHub (Light)
  It would replace the current setting asciidoc.preview.useEditorStyle

That's a good idea 👍🏻