top-level annexes do not support explicit numbers

[ljharb]

top-level annexes do not support explicit numbers; if you need this, open a bug on ecmarkup

I need to make a change to Annex B, and I want it to show up as "B", and I don't want to have to add a placeholder for "A".

[ljharb]

Actually, turns out i don't need Annex B for what I'm working on, but it still seems like a valid feature request :-)

[bakkot]

I'm not going to do work that isn't actually needed, but PRs welcome.