[Suggestion]: Disallow unclosed markup

[vhyrro]

This suggestion has already been accepted, but it is written here on the issues so that it can be tracked.

The second (and hopefully last) revision of the Norg specification will disallow unclosed markup. This was quite an easy change and isn't as scary as it seems initially:

1. The conditions for an opening modifier to exist are strict, this is by design. It's very difficult to unintentionally make something bold, for instance. Therefore, once the situation does arise, an error allows the user to act accordingly, for instance remedying the error by escaping the opening bold marker.
2. Unclosed markup massively complicates grammars for various common parser generators. The amount of combinations of sequentially closed + unclosed markup that plague our test suite is substantial.

All in all, this makes the format much more predictable in its behaviour, which is what Norg has always aimed to be.

[mrossinek]

All good 👍

Just leaving a note here to align with #33:
while & and $ super verbatim will have higher precedence than the backslash we must make sure that placing \& or \$ somewhere in the text still works as expected! Because with the current rules, the following is kind of annoying:

I love peanut butter & jam.

That & would result in an unclosed markup error...
So instead, one now always has to escape a standalone & or $ character... 😒

I love peanut butter \& jam.

Not sure if we can find a better solution 🤔

[vhyrro]

Hm, that's an interesting issue indeed. I think this will need some further considerations :)