New shorthand for branching over file stems

[wlandau]

Prework

• I understand and agree to tarchetypes' code of conduct.
• I understand and agree to tarchetypes' contributing guidelines.
• New features take time and effort to create, and they take even more effort to maintain. So if the purpose of the feature is to resolve a struggle you are encountering personally, please consider first posting a "trouble" or "other" issue so we can discuss your use case and search for existing solutions first.

Proposal

From ropensci/targets#136, it takes two targets to branch over a file stem.

list(
  tar_target(paths, c("a.csv", "b.csv")),
  tar_target(files, paths, pattern = map(paths), format = "file")
)

We should define a new target archetype to condense this down to one line.

tarchetypes::tar_files(files, c("a.csv", "b.csv"))