Add warning if Embeddium mod file is opened directly

[embeddedt]

This mostly involves copying CaffeineMC/sodium-fabric#2250, but the build.gradle change will need to be done differently on 1.18+ as we use NeoGradle instead of Loom.

[embeddedt]

There has never been a single report of anyone trying to open the mod file directly, and the Gradle configuration would have to be done three times: once for ForgeGradle (1.20.1), once for NeoGradle (1.20.6), and once for Loom (both versions). I simply don't have the interest to do that, so this issue will be closed. However, I'll happily accept a PR if someone does wish to add this.