Markup: replace `<emu-normative-optional>` tags with `normative-optional` attribute

[michaelficarra]

<emu-normative-optional> is not a supported feature of ecmarkup. Ecmarkup has official support for marking clauses as normative optional via the normative-optional attribute. You can read the ecmarkup docs at https://tc39.es/ecmarkup/. We would like to remove special handling of this tag from ecmarkup. It is only used by 402 as far as we aware.

See also tc39/ecmarkup#641.