Add missing import extension, check with lint

Some pure-ESM processors (eg, `tsc` with `moduleResolution: 'nodenext')
require extensions on all imports. #4778 added two import-less
extensions. This PR fixes them and adds linting (from an
already-installed plugin) to catch future errors. Test execution does
not appear to require these extensions so we only check non-test files
(there's another 17 errors to fix if you want to check tests too).

.eslintrc.cjs

@@ -4,8 +4,14 @@ module.exports = {
   parserOptions: {
     project: './tsconfig.json',
   },
-  extends: ['eslint:recommended', 'standard', 'prettier', 'plugin:@typescript-eslint/recommended'],
-  plugins: ['@typescript-eslint'],
+  extends: [
+    'eslint:recommended',
+    'standard',
+    'prettier',
+    'plugin:@typescript-eslint/recommended',
+    'plugin:import/typescript',
+  ],
+  plugins: ['@typescript-eslint', 'import'],
   rules: {
     'no-empty': 'off',
     'no-console': 'off',
@@ -94,6 +100,11 @@ module.exports = {
         'promise/param-names': 'off',
       },
     },
+    {
+      files: ['**/*.ts'],
+      excludedFiles: ['**/{test,tests,testing}/**/*.{ts,js}', '*.{spec,test}.{ts,js}'],
+      rules: { 'import/extensions': ['error', 'ignorePackages'] },
+    },
   ],
   ignorePatterns: [
     'dist',

packages/utils/src/Path.ts

@@ -1,4 +1,4 @@
-import { Maybe } from './types';
+import { Maybe } from './types.js';
 
 export interface Path {
   readonly prev: Path | undefined;

packages/utils/src/jsutils.ts

@@ -1,4 +1,4 @@
-import { MaybePromise } from './executor';
+import { MaybePromise } from './executor.js';
 
 export function isIterableObject(value: unknown): value is Iterable<unknown> {
   return value != null && typeof value === 'object' && Symbol.iterator in value;