Meta: bump ecmarkup to 15.0.0 and apply new formatting/lint rules

[bakkot]

Depends on https://github.com/tc39/ecma262/pull/2896, and is based on that branch.

I recommend reviewing commits individually, and commits should land individually.

The "Markup: replace HTML entities" commit was generated by running npm run format; no manual changes were applied. Sidebar: we sure do have a lot of fancy quote marks.

The "Markup: mark certain non-obvious variable declarations" commit is necessary to get the linter's new "variables must be defined" lint to pass; the ten lines it touches make use of variables in non-obvious ways that I didn't want to teach the linter about.