add CI by GitHub Actions

[hokkai7go]

I think ruby-dicom is a useful gem. I thought it would be good if it was compatible with Ruby3. However, although there is test code, there is no CI. I added CI using GitHub Actions to make it easier to check the impact of changes.

GitHub Actions can be used for free in the Public Repository. https://docs.github.com/ja/billing/managing-billing-for-your-products/about-billing-for-github-actions

I used setup-ruby to set up the Ruby environment in GitHub Actions. I tried to set up the environment with Ruby1.9.3 according to REQUIREMENTS, but gave up and decided to use Ruby2.2 as described in gemspec.

Here are the test results.

I don't have the results to show how many specs were failing before this work. Please let me know if there are any changes that need to be made.