Build: Enable GitHub Actions based CI

[Johni0702]

Also removes 1.15 because of a bug in archloom (1).
Diff between 1.16.2-forge before and after the removal shows no significant
changes:

diff -r org/main/java/gg/essential/universal/UGraphics.java versions/1.16.2-forge/build/preprocessed/main/java/gg/essential/universal/UGraphics.java
55,58c55,58
< //$$ import net.minecraft.client.renderer.Matrix3f;
< //$$ import net.minecraft.client.renderer.Matrix4f;
< //$$ import net.minecraft.client.renderer.Vector3f;
< //$$ import net.minecraft.client.renderer.Vector4f;
---
> //$$ import org.lwjgl.util.vector.Matrix3f;
> //$$ import org.lwjgl.util.vector.Matrix4f;
> //$$ import org.lwjgl.util.vector.Vector3f;
> //$$ import org.lwjgl.util.vector.Vector4f;
diff -r org/main/kotlin/gg/essential/universal/vertex/VanillaVertexConsumer.kt versions/1.16.2-forge/build/preprocessed/main/kotlin/gg/essential/universal/vertex/VanillaVertexConsumer.kt
9,12c9,12
< //$$ import net.minecraft.client.renderer.Matrix3f
< //$$ import net.minecraft.client.renderer.Matrix4f
< //$$ import net.minecraft.client.renderer.Vector3f
< //$$ import net.minecraft.client.renderer.Vector4f
---
> //$$ import org.lwjgl.util.vector.Matrix3f
> //$$ import org.lwjgl.util.vector.Matrix4f
> //$$ import org.lwjgl.util.vector.Vector3f
> //$$ import org.lwjgl.util.vector.Vector4f
diff -r org/main/kotlin/gg/essential/universal/wrappers/message/UTextComponent.kt versions/1.16.2-forge/build/preprocessed/main/kotlin/gg/essential/universal/wrappers/message/UTextComponent.kt
182c182
<         //$$     is String -> StringTextComponent(hoverValue as String)
---
>         //$$     is String -> TextComponentString(hoverValue as String)
185c185
<         //$$     else -> StringTextComponent(hoverValue.toString())
---
>         //$$     else -> TextComponentString(hoverValue.toString())

To be squashed on merge.

Publish doesn't work yet due to secrets not being set up, but otherwise should be good to review.

[DJtheRedstoner]

The reason this caused a maven publish with the branch name HEAD is that pull_request triggers are based on the merge commit for the PR, so the branch name isn't associated. We'll probably need some override (env variable?) in EGT that actions can pass through from github.event.pull_request.head.ref.

[DJtheRedstoner]

We should also cache the loom-cache folders in the project, this contains remapped mods and MC versions with AW/ATs applied (only the first is relevant to our mods though). See Sk1erLLC/Patcher#72

[Johni0702]

The reason this caused a maven publish with the branch name HEAD is that pull_request triggers are based on the merge commit for the PR, so the branch name isn't associated. We'll probably need some override (env variable?) in EGT that actions can pass through from github.event.pull_request.head.ref.

Hm, looks like that's the case. I expected it to use something like refs/pull/55/merge cause that's what it fetches and checks out, but I suppose checking out an arbitrary ref is more like checking out a specific commit than a branch, and git rev-parse can't know you did it.
Luckily we already have an override in EGT because the old CI used one all the times.