fix stryker config

stryker.conf.js

@@ -0,0 +1,17 @@
+'use strict';
+
+/**
+ * @type {import('@stryker-mutator/api/core').StrykerOptions}
+ */
+module.exports = {
+  mutator:          'javascript',
+  packageManager:   'yarn',
+  reporters:        [
+    'clear-text',
+    'progress'
+  ],
+  testRunner:       'command',
+  transpilers:      [],
+  coverageAnalysis: 'all',
+  mutate:           [ 'index.js' ]
+};

stryker.conf.json

@@ -1,13 +0,0 @@
-{
-  "$schema": "https://raw.githubusercontent.com/stryker-mutator/stryker/master/packages/api/schema/stryker-core.json",
-  "mutator": "javascript",
-  "packageManager": "yarn",
-  "reporters": [
-    "clear-text",
-    "progress"
-  ],
-  "testRunner": "command",
-  "transpilers": [],
-  "coverageAnalysis": "all",
-  "mutate": ["index.js"]
-}