Set up permissions for GitHub Workflows

[joycebrum]

Closes #216

Changes

• Set main.yml top level permission to contents: read

Test

I've tested and the workflow ran with success https://github.com/joycebrum/antlr3/actions/runs/4246430550

[joycebrum]

Hi!

I'm writing to follow up on your review in this pull request. I noticed that you haven't left any feedback yet.

I'm happy to answer any questions you have about my changes. I'm also available to chat if you'd like to discuss my changes in more detail.

If you have any concerns about my changes, please let me know so that I can address them.

If the PR goes idle for the next 2 months, I'll close it. If you'd rather keep it open as "not planned", just let me know and I'll keep it open even if it gets idle again.

Thanks!