Fix NPM warnings, round 2 (#1332)

Co-authored-by: Melroy van den Berg <[email protected]>

assets/app.js

@@ -1,6 +1,6 @@
 import './bootstrap.js';
 import './styles/app.scss';
-import './utils/popover';
+import './utils/popover.js';
 import '@github/markdown-toolbar-element';
 import { Application } from '@hotwired/stimulus';
 

eslint.config.mjs

@@ -0,0 +1,67 @@
+import js from "@eslint/js";
+import globals from "globals";
+import stylistic from "@stylistic/eslint-plugin";
+
+export default [
+    {
+        ignores: ["*", "!assets"],
+    },
+    js.configs.recommended,
+    {
+        plugins: {
+            "@stylistic": stylistic,
+        },
+        languageOptions: {
+            globals: {
+                ...globals.browser,
+                ...globals.node,
+                ...globals.es2021
+            },
+            ecmaVersion: "latest",
+            sourceType: "module",
+        },
+        rules: {
+            "@stylistic/indent": ["error", 4],
+            "@stylistic/linebreak-style": ["error", "unix"],
+            "@stylistic/eol-last": ["error", "always"],
+            "@stylistic/arrow-parens": ["error", "always"],
+            "@stylistic/brace-style": ["error", "1tbs"],
+            "@stylistic/comma-dangle": ["error", "always-multiline"],
+            "@stylistic/comma-spacing": ["error"],
+            "@stylistic/keyword-spacing": ["error"],
+            "@stylistic/no-multiple-empty-lines": ["error", {
+                max: 2,
+                maxEOF: 0,
+                maxBOF: 0,
+            }],
+            "@stylistic/no-trailing-spaces": ["error"],
+            "@stylistic/no-multi-spaces": ["error"],
+            "@stylistic/object-curly-spacing": ["error", "always"],
+            "@stylistic/quotes": ["error", "single", {
+                avoidEscape: true,
+            }],
+            "@stylistic/semi": ["error", "always"],
+            "@stylistic/space-before-blocks": ["error"],
+            "@stylistic/space-in-parens": ["error", "never"],
+            camelcase: ["error", {
+                ignoreImports: true,
+            }],
+            curly: ["error", "all"],
+            eqeqeq: ["error", "always"],
+            "no-console": ["off"],
+            "no-duplicate-imports": ["error"],
+            "no-empty": ["error", {
+                allowEmptyCatch: true,
+            }],
+            "no-empty-function": ["error", {
+                allow: ["arrowFunctions", "constructors"],
+            }],
+            "no-eval": ["error"],
+            "no-implicit-coercion": ["error"],
+            "no-implied-eval": ["error"],
+            "prefer-const": ["error"],
+            "sort-imports": ["error"],
+            yoda: ["error", "always"],
+        },
+    }
+];