Convert Velocity buildscripts to Kotlin DSL (#918)

Spiritually indebted to #518 and @alexstaeding.

There's a minor break - we're going up to 3.2.0-SNAPSHOT as the API now compiles against Java 11. But this is more academic in practice.

native/build.gradle.kts

@@ -0,0 +1,18 @@
+plugins {
+    `java-library`
+    `maven-publish`
+}
+
+license {
+    header(project.rootProject.file("HEADER.txt"))
+}
+
+val guavaVersion: String by project.extra
+val nettyVersion: String by project.extra
+val checkerFrameworkVersion: String by project.extra
+
+dependencies {
+    implementation("com.google.guava:guava:${guavaVersion}")
+    implementation("io.netty:netty-handler:${nettyVersion}")
+    implementation("org.checkerframework:checker-qual:${checkerFrameworkVersion}")
+}