Pending TODOs for Template Automation

Just parking these here (_and add your own, plz_).
A TODO list for templates, template scripts, and other test file & template-related things.

- [ ] Macro-tize or script filter the `additional_tests.json` portion. Having to manually read `additional_cases` into templates is a PITA.
- [ ] Macro-tize error checking syntax see the commented out Python macro [here](https://github.com/exercism/r/blob/main/config/generator_macros.j2#L25-L33)
- [ ] Find an auto-formatter for JinJa files, so that they are clean, and we all agree on how they look
- [ ] Find a linter (long shot here) for JinJa files, so we are always clear on how to do things.
- [ ] Consider adding other helpful info to the test header file. Do we care what version of R these were done against? What about the version of Tidyverse? Other things we could add or annotate to help with all of this?
- [ ] Consider adding a master template that would cover 80-90 percent of the cases.
- [ ] Consider adding documentation for new contributors to make this process easier. Python docs [here](https://github.com/exercism/python/blob/main/docs/GENERATOR.md). Most likely needs updating/massaging.
- [ ] Consider adding test regeneration to CI (but only if it doesn't take forever).
- [ ] Better debug actions when rendering goes bad
- [ ] Some additional (potential) filters or macros for processing errors, highly nested data, and other JSON --> R data structure conversions.