fileSummary support directory inputs

[andersone1]

Summary

This PR updates fileSummary() so it can accept either a single file path or a directory path.

What Changed

• Added directory handling in fileSummary() using fs::is_dir().
• When a directory is supplied, fileSummary() now:
  • Lists immediate files in that directory.
  • Excludes subdirectories.
  • Runs fileSummary() on each file.
  • Invisibly returns a named list of per-file summaries.
• Kept existing single-file behavior intact.
• Updated error text to reflect the new input contract (single file or directory path).
• Updated documentation (R/fileSummary.R, man/fileSummary.Rd) including examples.
• Added tests for directory input and updated existing input-validation expectations.

Why

This removes the need for external loops like:
purrr::walk(list.files(...), ~ review::fileSummary(.file = .x))
and makes directory-level summaries a built-in behavior of fileSummary().