GH Action triggered on merged to other repositories

[jdebacker]

In order to ensure that the reform files here work with the latest versions of other packages (e.g., Tax-Calculator), it would be nice to set up a GH Action that triggers the unit tests run in this repo with the latest version of those packages.

I'm not sure if a test on the merge to master or a test on the release of a new package version is more appropriate.

cc @MattHJensen

[MattHJensen]

Testing on merge to master would allow examples to be brought up to date before a new release is issued; that seems preferable to me.