Add outline plugin

[yhatt]

Add a markdown-it plugin called outline plugin, to be recognized slide pages by VS Code's outline pane. It can jump to the other slide page easily.

In addition, it would allow to fold the code of slide contents per page.

The added plugin will manipulate parsed tokens to add the invisible header token before each slides as the virtual <h0> element. They have a line data mapped to the divider, and VS Code's Markdown support would process them well.

Experimental?

At first, we want to release this as an experimental feature because of some restrictions:

• VS Code disallows to show custom text in outline pane, so the top level list would fill by --- in most cases. We are not yet sure if it is good or bad.
• Outline pane will show the sorted outline by the content of corresponding line. It has no problem in typical usage, but it may break order of slides when Markdown used some different dividers.
  • 👉 It seems that it resolves just by changing sort setting from context menu: "Sort By: Position".
• headingDivider global directive is not yet supported. It may require a feedback to Marpit.
  • 👉 Updated Marpit has improved line mapping in heading divider.

UPDATE: Our fears almost could resolve. We will ship as non-experimental feature with add notes to README.md.

We might add the separated panel to show detailed outline by depending on the feedback, just like vscode-reveal.