Don't call formatter on directories

[edennis]

This came up today with a custom formatter that we have for json files. Vscode's ElixirLS created a directory called "apps/our_app/.elixir_ls/build/test/lib/phoenix/priv/templates/phx.gen.json" and running mix format resulted in the following error:

** (File.Error) could not read file "/Users/edennis/sandbox/...": illegal operation on a directory
    (elixir 1.13.4) lib/file.ex:355: File.read!/1
    (mix 1.13.4) lib/mix/tasks/format.ex:563: Mix.Tasks.Format.format_file/2
    (elixir 1.13.4) lib/task/supervised.ex:89: Task.Supervised.invoke_mfa/2
    (elixir 1.13.4) lib/task/supervised.ex:34: Task.Supervised.reply/4
    (stdlib 3.17.2.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

The fact that we're accidentally formatting files from ElixirLS is of course an issue of ours, but with the exception of :stdin, I'd say we'd only want to ever try to format regular files.

[whatyouhide]

Nice catch, I think it makes sense. 💯

[josevalim]

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