Add GitHub Action that compiles project

[jakejack13]

This PR adds a CI (continuous integration) pipeline via GitHub Actions that will check whether the project compiles. Here's a breakdown of how it works:

1. The action triggers on either a commit to the main branch (push), a pull request (pull_request), or by manually being invoked on GitHub (workflow_dispatch)
2. The action clones the repository, downloads Java, and then builds the project with Gradle.
3. The outputs show up next to the commit or on the pull request. It will be either a green check (everything compiles!) or a red X (sad). You can see this in action on this very PR under the new checks menu! You'll see "All checks have passed. 1 successful check"

This will help give a visual indication of if a PR is ready to be merged or if main is currently valid without having to check out the branch and do it manually. It's a very simple check for now but you can expand on it with all sorts of things.

[JacquelineNi]

yummy code :3