[orx-easing] Add Serializable annotations

orx-easing/build.gradle.kts

@@ -1,5 +1,9 @@
 plugins {
     org.openrndr.extra.convention.`kotlin-multiplatform`
+    // kotlinx-serialization ends up on the classpath through openrndr-math and Gradle doesn't know which
+    // version was used. If openrndr were an included build, we probably wouldn't need to do this.
+    // https://github.com/gradle/gradle/issues/20084
+    id(libs.plugins.kotlin.serialization.get().pluginId)
 }
 
 kotlin {
@@ -12,6 +16,7 @@ kotlin {
                 implementation(libs.openrndr.draw)
                 implementation(libs.openrndr.filter)
                 implementation(libs.kotlin.reflect)
+                implementation(libs.kotlin.serialization.core)
             }
         }