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.

settings.gradle.kts

@@ -0,0 +1,9 @@
+rootProject.name = "velocity"
+include(
+        "api",
+        "proxy",
+        "native"
+)
+findProject(":api")?.name = "velocity-api"
+findProject(":proxy")?.name = "velocity-proxy"
+findProject(":native")?.name = "velocity-native"