Rebase to the markdown extension code base

[joaompinto]

While planning for #11 I have noted that most of the VSCode markdown features make sense for asciidoc. As such, it will be more efficient/productive/maintainable to reuse as much as possible from the markdown code base, adapting just what is required for the different render engine (asciidoctor.js replacing markdown-it)

https://github.com/Microsoft/vscode/tree/master/extensions/markdown-language-features

Because this is a major feature/change, it will be worked in a new branch:
https://github.com/joaompinto/asciidoctor-vscode/tree/markdown-rebase

[joaompinto]

Instead of the branch, we will be using a new repository https://github.com/joaompinto/vscode-asciidoctor, this will allow for better tracking issues in relation to a new code base, and also getting a more used naming convention for the name (vscode-extension).

[joaompinto]

Closing this issue for now, the built-in markdown depends on insiders-only API features and there is no plan to have such features available for general use.