use ES modules

.config

@@ -1,4 +1,3 @@
 repo-owner = sanctuary-js
 repo-name = sanctuary-descending
 contributing-file = .github/CONTRIBUTING.md
-module-type = commonjs

.eslintrc.json

@@ -1,19 +1,9 @@
 {
   "root": true,
   "extends": ["./node_modules/sanctuary-style/eslint.json"],
-  "parserOptions": {"ecmaVersion": 2020},
+  "parserOptions": {"ecmaVersion": 2020, "sourceType": "module"},
   "globals": {"globalThis": "readonly"},
   "rules": {
     "comma-dangle": ["off"]
-  },
-  "overrides": [
-    {
-      "files": ["index.js"],
-      "globals": {"__doctest": "readonly", "define": "readonly", "module": "readonly", "require": "readonly", "self": "readonly"}
-    },
-    {
-      "files": ["test/**/*.js"],
-      "parserOptions": {"sourceType": "module"}
-    }
-  ]
+  }
 }