The formatter uses `devShell.packages` for its path which is not
available when devShell is null. A default value of empty list should be
used when devShell is null.
Using a submodule for devShell removes the need to make every option
nullable and the need to check all of them. By using nullOr submodule,
we can tell if the value has been set and have default values for
options.
This also enables enabling a devShell with no options set.
