Mark Base64 API with ExperimentalEncodingApi

libraries/stdlib/js-ir/build.gradle.kts

@@ -143,6 +143,7 @@ tasks.withType<KotlinCompile<*>>().configureEach {
         "-opt-in=kotlin.RequiresOptIn",
         "-opt-in=kotlin.ExperimentalUnsignedTypes",
         "-opt-in=kotlin.ExperimentalStdlibApi",
+        "-opt-in=kotlin.io.encoding.ExperimentalEncodingApi",
         "-XXLanguage:+RangeUntilOperator",
     )
 }