Add CI via Github Actions

[MCOfficer]

This PR adds an action run that generates a signed .jar and .exe on every commit.

In order to make the CI use your certificate, you need to:

• Encrypt your keystore and replace keystore.gpg with it. See steps 1-4
• Under Settings -> Secrets, add
  • CERT_ALIAS - your certificate's alias in the keystore
  • KEYSTORE_ENCRYPTION_PASS - the passphrase used to encrypt the keystore
  • KEYSTORE_PASS - the keystore's password