Support referencing algorithm steps

[bakkot]

Currently emu-xref allows linking to specific algorithms, but sometimes (especially in Annex B) we want to talk about specific steps of those algorithms. There's no way to do that automatically, which means we write out step numbers, and those inevitably get stale. It would be good if there were some way to give names to certain algorithm steps and then refer to those names and have that be translated to the appropriate step number.

[ExE-Boss]

See also: tc39/ecmarkdown#56.