Allow `dhall format` to take multiple files as arguments

[srid]

cf. numtide/treefmt-nix#49

Would you be open to a PR for this?

[SiriusStarr]

dhall format does take multiple files as arguments, unless I am not understanding what you mean.

Usage: dhall format [--transitive] [--check] [FILES]

  Standard code formatter for the Dhall language

Available options:
  --transitive             Modify the input and its transitive relative imports
                           in-place
  --check                  Only check if the input is formatted
  FILES                    Read expression from files instead of standard input
  -h,--help                Show this help text

[srid]

Ah, yes, as of 1.39.0. Thanks

#2169