Add dhall formatter

[srid]

https://github.com/dhall-lang/dhall-haskell/pull/137

It takes one file at a time in the command line, though. Is this mode supported by treefmt?

dhall format --inplace ./foo.dhall

[zimbatm]

You'll have to wrap it with a shell script that does something like this:

set -eu
for file in "$@"; do
  dhall format --inplace "$file"
done

Or fix upstream so it can take multiple files.

Per the formatter spec the formatter should also only write back the file if the content has changed.