chore: switch to eslint + upgrade packages

.eslintrc.cjs

@@ -0,0 +1,39 @@
+/* eslint-env node */
+module.exports = {
+  root: true,
+  extends: [
+    'eslint:recommended',
+    'plugin:@typescript-eslint/recommended',
+    'prettier',
+  ],
+  overrides: [
+    // TypeScript
+    {
+      files: ['**/*.ts'],
+      parser: '@typescript-eslint/parser',
+      plugins: ['@typescript-eslint'],
+      parserOptions: {
+        project: true,
+        tsconfigRootDir: __dirname,
+      },
+      rules: {
+        // Only disabling this because this code is very old but also battle-tested,
+        // and coming up with clever typings may break for consumers.
+        '@typescript-eslint/no-explicit-any': 'off',
+        // We use `require` for loading the `package.json` file.
+        '@typescript-eslint/no-var-requires': 'off',
+      },
+    },
+    // Tests
+    {
+      files: ['**/__tests__/*.test.ts'],
+      rules: {
+        // Unused vars are used for dependency resolution.
+        '@typescript-eslint/no-unused-vars': 'off',
+
+        // Some types are useful for testing.
+        '@typescript-eslint/ban-types': 'off',
+      },
+    },
+  ],
+}