Add metadata to collapse "more details" and "status" sections

[jyasskin]

In https://w3ctag.github.io/explainers/, I'd like the "More details about this document" and "Status of this document" to start collapsed, so that people reading the document get right to the advice. I could do that by locally overriding header.include, but then we don't get any future updates to the w3c default.

My theory is that the way to do this is to add a metadata "Collapse-Metadata: boolish", which defines an empty [COLLAPSE-METADATA] text macro, and then use that in w3c/header.include to remove [open] from the More details <details> and wrap the Status section with a <details>. I'll start on that, but I wanted to file an issue to see if there's a better way.

[jyasskin]

It turns out that pubrules currently rejects documents with these changes. I've filed w3c/specberus#1919 and w3c/specberus#1920 to try to loosen that up.

I also wrote a change to implement this, at main...jyasskin:bikeshed:collapse-metadata, but I'll wait to send it until I know whether pubrules will accept the change.

[tabatkins]

We can make this a little simpler, fwiw: set COLLAPSE-METADATA to "open" when the pref is true. Then you can just write <details [COLLAPSE-METADATA?] ...> in the boilerplate, rather than duplicating the element.

[jyasskin]

I think that's [OPEN-METADATA] that's "open" when Collapse-Metadata is no, so that it can also be used in include-if. Perhaps that means that it should be Open-Metadata defaulting to yes?