Make copyrightYear and releaseLabel optional

[cmoesel]

Description: Make copyrightYear and releaseLabel configuration properties optional since not all publishing tools required them. Warn when they are missing, however, since the HL7 IG Publisher does require them.

Testing Instructions:

1. Create a new test FSH project (e.g., sushi init TestPR)
2. Run this branch's SUSHI against the test project (e.g., ts-node src/app.ts build /path/to/TestPR) and note that there are no errors or warnings
3. Edit the test project's sushi-config.yaml to delete the line that sets copyrightYear
4. Run this branch's SUSHI against the test project again and note that there is a warning (but no errors)
5. Edit the test project's sushi-config.yaml to delete the line that sets releaseLabel
6. Run this branch's SUSHI against the test project again and note that there are now two warnings (but no errors)
7. Add FSHOnly: true to the project's sushi-config.yaml
8. Run this branch's SUSHI against the test project again and note that there are no warnings or errors

Related Issue: #1625