Add support for a translation context

[mgeisler]

Yeah, we haven't added that, though I suspect it can follow a similar pattern to how we're doing comments.

So probably a sketch of supporting this would be:

• Define another HTML comment directive to let authors specify the ctxt, similar to what we're doing here

• Collect and thread that state when we're gathering groups for translation.

• Finally, pass the ctxt here and call the builder method for setting the ctxt.

So I think this would be fairly straightforward extension.

Originally posted by @dyoo in google/comprehensive-rust#1973 (comment)

[mgeisler]

This is about adding support for the translation context.