Feature variants (#376)

Migrate from buildSrc to build-logic. Setup feature variants.

build-logic/orx-variant-plugin/build.gradle.kts

@@ -0,0 +1,17 @@
+plugins {
+    `kotlin-dsl`
+}
+
+repositories {
+    mavenCentral()
+}
+
+
+gradlePlugin {
+    plugins {
+        create("orxVariants") {
+            id = "orx-variant"
+            implementationClass = "org.openrndr.extra.variant.plugin.VariantPlugin"
+        }
+    }
+}