Add GitHub Actions Workflow to build and publish

[duhow]

• Runs build.sh command on PR and on push commit to master branch
• If you generate a new GitHub Release (or git tag), the packages will self-build and publish in the Release.
  (needs write permission in Workflow permissions)

Feel free to squash for merging :)

[duhow]

I'll also be working in creating a Docker image to run n80 for other repositories.

[Konamiman]

Hi, thanks for your contribution, but after merging #5 some things have changed:

• build.sh is now in the root of the repository and will build all three programs (N80, LK80, LB80), placing the results in the Release directory of each program.
• Same for zip.sh, will generate the zip files in the Release/zips directory of each program.
• The .NET SDK 7 is now required for the build process (the resulting programs continue to target the .NET 6 runtime, though; I prefer to target LTS releases only).

Could you please adjust your pull request (and I guess #4 will also require changes) to take this in account? Thank you!