Meta: bump ecmarkup to 15.0.0

[bakkot]

This bumps ecmarkup, which now includes a lint for variables being declared before being used. I've marked two sites which appear to be introducing a variable as doing so, and corrected a typo in a third place.

There's another place which refers to a variable which doesn't appear to be defined anywhere, but I couldn't figure out what that was supposed to be doing, so I've left it alone. That should cause this to fail CI. Please feel free to push up a commit fixing that issue before landing this.

Edit: oh, looks like lint rules aren't enforced in CI, so it's not failing the build. You might want to do that - it's just a matter of adding --strict to the build command.

(Ignore the branch name; I was originally intending for this to include a github action to publish the biblio but I'll do that as a followup.)

[gibson042]

Can this also resolve #634 via https://github.com/tc39/ecmarkup/pull/481 ?

[bakkot]

Running npm x emu-format -- --write 'spec/*' after this PR will translate almost all HTML entities into their corresponding codepoints, yes. It will also change a number of other formatting details to line up with the conventions used in ecma262.

[ryzokuken]

@gibson042 can we go ahead and merge this?