[JS IR] add a flag which prepends a custom prefix to the per-module .js files

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JSCompilerArguments.kt

@@ -157,6 +157,9 @@ class K2JSCompilerArguments : CommonCompilerArguments() {
     @Argument(value = "-Xir-per-module", description = "Splits generated .js per-module")
     var irPerModule: Boolean by FreezableVar(false)
 
+    @Argument(value = "-Xir-per-module-prefix", description = "Adds a custom prefix to the splitted js files")
+    var irPerModulePrefix: String? by NullableStringFreezableVar(null)
+
     @Argument(
         value = "-Xinclude",
         valueDescription = "<path>",