Clean repo for open source release

Remove build artifacts, dev tool configs, unused dependencies,
and third-party source dumps. Add proper README, update .gitignore,
clean up Makefile.

settings.gradle

@@ -0,0 +1,13 @@
+pluginManagement {
+    repositories {
+        gradlePluginPortal()
+        maven {
+            name = 'MinecraftForge'
+            url = 'https://maven.minecraftforge.net/'
+        }
+    }
+}
+
+plugins {
+    id 'org.gradle.toolchains.foojay-resolver-convention' version '0.7.0'
+}