New feature: mandatory references

[acristu]

@TomasHubelbauer

Hi, after the big rewrite you mentioned in #11 , would you consider a new feature called perhaps "mandatory references":

• have a configuration parameter mandatoryReferencesRegex=Prefix.*
• report errors on all headings that match the mandatoryReferencesRegex but are not linked-to from anywhere

[TomasHubelbauer]

This sounds interesting. I will develop it. I will update this issue when I know more about the timeline.

[TomasHubelbauer]

I have had some time recently to fix some long overdue issues which were introduced by the VS Code API switching from a flat list of MarkDown header symbols to a tree of those. I have just released a new version, 13.0.0, which contains those fixes.

With that out of the way, I am now free to take a look at this feature in the next few days. I will update this issue when I've implemented it so that you can test it out and provide feedback.