Fix Gradle and add build workflow

[object-Object]

Changes:

• Fix several errors preventing Fabric:runClient and Forge:runClient from running.
• Add and commit CHANGELOG.md instead of creating a private changelog file for each release.
• Remove the keys.properties file, instead getting tokens from environment variables.
• Run the datagen.
• Add a GitHub Actions workflow to build and test the mod (copied from HexDummy; release workflow TODO).
• Bump version from 0.3.0-3-skyevg-unofficial to 0.3.0-4.