CI formatting check

Currently, https://github.com/exercism/dart/blob/main/.github/workflows/test.yml checks the formatting before running the test suite, potentially failing before the test suite is checked. This should instead be done as a separate GH action.

Any of the three dart files (stub, test suite, or example solution) could be formatted incorrectly so it's unclear which file is the issue from the CI logs. The GH action should specify if possible which file was reformatted and ideally what the change is. `dart format` can show the formatted code for the changed file so perhaps we can compare the formatted code to the original file and display the difference. Surfacing that difference should reduce confusion since we're using dart format with an override for line length. I ran afoul of that early on since locally `dart format` was working fine as far as I could tell.