in `scribble/lp2`, `chunk` should use `#:tag-prefix`

[mbutterick]

Consider a project that gathers multiple scribble/lp2 files into a single document. For TOC / navigation purposes, the names of the chunk blocks are treated like section titles. But unlike section, subsection, et al., they don't obey the #:tag-prefix setting, which means that to make things index correctly, you have to manually assign unique names to every chunk in the project.