Switch over to Error Prone (now with NullAway!)

There is one major change: we now have a separate artifact for the annotation processor.

As for NullAway, we are currently exempting the clientbound join game/respawn packets. They are ugly and need to be refactored.

annotation-processor/build.gradle

@@ -0,0 +1,35 @@
+plugins {
+    id 'java-library'
+    id 'maven-publish'
+    id 'checkstyle'
+}
+
+apply plugin: 'org.cadixdev.licenser'
+apply from: '../gradle/checkstyle.gradle'
+apply from: '../gradle/publish.gradle'
+apply from: '../gradle/errorprone.gradle'
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_11
+    targetCompatibility = JavaVersion.VERSION_11
+}
+
+license {
+    header = project.rootProject.file('HEADER.txt')
+}
+
+dependencies {
+    implementation project(':velocity-api')
+}
+
+test {
+    useJUnitPlatform()
+}
+
+publishing {
+    publications {
+        mavenJava(MavenPublication) {
+            from components.java
+        }
+    }
+}