Meta: bump ecmarkup to 6.1.0

[bakkot]

Notable changes:

• add support for normative optional attribute ecmarkup#292, which also entailed dropping the inline CSS here (as promised)
• css: add overflow:auto to notes ecmarkup#291, which fixes an overflow observable in e.g. in RegExp.prototype [ @@split ]
• dynamically add copyable step numbers ecmarkup#290, which makes it so that if you copy-paste an algorithm step (by triple-clicking to select the whole line) it will include the step number

I've also added a dfn to "Normative Optional", so that you can click on the label in the heading of such clauses to be taken to its definition.