Remove shortcut-style links from headers in ToC

[BeeeWall]

Currently, the regex used to match links does not match shortcut-style reference links (ones with no reference after them, such as [link]). This PR fixes that, by making the second set of brackets optional in the regex.

This also adds tests for those and collapsed-style links ([link][]).

Note: for whatever reason, the tests won't run for me (even before I added the new ones, so I don't believe it's an issue caused by my changes), so I can't run the tests locally. But since the PR will automatically run them that should work out still.

[BeeeWall]

How did changing the comments cause it to fail-

The test failure just says it timed out (and on a test unrelated to where the comments are located, I think). So maybe that's a one-time failure? Because I can't see how a comment change like that would actually break it.

[yzhang-gh]

Thanks. But I am wondering whether we should do this as [] is more likely to be literally just square brackets rather than a reference link when it appears in a heading. An example is, when people use 1. in a heading, they don't mean it being a Markdown list.

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The test failure just says it timed out

It is not a problem with your code. From time to time it fails.

[BeeeWall]

I thought about that, but it looks like brackets being in the headers breaks them. Tested using both Markdown Preview Advanced and the default Markdown previewer.

[yzhang-gh]

I see. If you have reference link definitions (e.g. [Test]: example.com), the [Test] in the heading will be rendered by the Markdown parser, otherwise it is left as is ([Test]).

So what we can do is to only remove brackets of [foo] if there exists [foo]: bar.

Personally, I would suggest not using the shortcut syntax. It is prone to cause problems.

[yzhang-gh]

Thanks for your contribution. For now, let's just keep it simple (doesn't recognize shortcut-style reference links).