Make it easier to add shells to shell.rs

[workingjubilee]

We discussed and there was an understanding gap that has been bridged, in terms of smoothing out the code. The specific requests were in service of a goal I did not understand (and initially did not inquire too deeply about). As I now understand the higher-level goal (difficult-to-itemize improvements in clarity), I can draft some commits in service of that.

Originally posted by @workingjubilee in #2387 (comment)

Documenting the issue here so I don't lose track of it before I open a new PR. Goal is to rework the helper fns in the module to be more generalized and composable where possible.