Create a dependabot configuration file

[devoncarew]

• create a dependabot configuration file

[mit-mit]

What deps is this intended to move forward?

Just the Dart example directory? Then we need the beta opt-in flag also, I think?

[athomas]

Also directory: "/" would have to point to example then. However, the example only has a single git dependency, so dependabot wouldn't do much?

[devoncarew]

This is just intended to update the deps that we use in github actions when testing this repo. That's currently just the actions/checkout dep in the .github/workflows/dart.yml file.