As discussed during this week's call, this adds @attributes to expression and markup with runtime impact.

For the syntax & data model, this is mostly a rebase of #450 that leaves out any discussion of what attributes might be good for, while noting that "Attributes are reserved for use by future specification versions."

For formatting, the language is rather more forbidding:

Attributes MUST NOT have any impact on message formatting, and MUST NOT be available in the formatted output. Variables in attribute values are not resolved, and no errors are emitted for them.

Closes #453, as using identifier brings in the same namespacing we're using for _options.

This also closes #587, as removing @ leaves the remaining symbols sorted in code point order.