RDFLib: Create github actions for pushes to main branch

[tian3rd]

It's a better practice to adopt CI/CD workflows early in developing a package.

Currently there's already some GitHub actions setup in the pods project, e.g., https://github.com/anusii/pods/blob/main/.github/workflows/dart.yml.

Also refer to https://docs.github.com/en/actions/quickstart for a jump start.

[tian3rd]

Closing criteria:

• [x] Finish setting a basic dart.yaml file for GitHub actions
• [x] Include the tests
• [x] (Optional) Include the formatting

[tian3rd]

For formatting, do not exit if it's not formatted well. Instead, format it automatically.

  - name: Verify formatting and apply linting
     run: dart format .