Replace prettier with stylistic, improve types, and add no-undef rule

tools/eslint.config.mjs

@@ -0,0 +1,20 @@
+import globals from 'globals';
+import { defineConfig, globalIgnores } from 'eslint/config';
+import { stylisticRules } from '../eslint.config.mjs';
+import stylistic from '@stylistic/eslint-plugin';
+
+export default defineConfig([
+    globalIgnores(['foundry/**/*']),
+    {
+        files: ['**/*.{js,mjs,cjs}'],
+        plugins: {
+            '@stylistic': stylistic
+        },
+        languageOptions: { globals: globals.node },
+        rules: {
+            'no-undef': 'error',
+            'no-unused-vars': 0,
+            ...stylisticRules
+        }
+    }
+]);