enable eslint for one particular rule (#6611)

So, Node requires generated ESM JS files need to have imports that end
with `.js` (unless they are importing the root or declared export from a
package... but for relative paths, this is a must). But we haven't quite
figured out a way to get tsc to force us to do this... so if we mess it
up, we only find out at runtime when trying to run a built ESM package.
This doesn't even necessarily show up in our tests (Jest does its own
thing a lot), and the new smoke-test doesn't cover all of our codebase.

So we set up eslint with a particular rule that looks for this. As it
happens, this rule doesn't check `import type`
(import-js/eslint-plugin-import#2270) but
fortunately `import type` doesn't matter to Node.

We don't generally enable eslint: there are plenty of rules that we
don't pass currently. We can consider adding some rules later as they
are found to be valuable. For now we just run this one rule!

.circleci/config.yml

@@ -58,6 +58,13 @@ jobs:
       - setup-node
       - run: npm run prettier-check
 
+  ESLint:
+    docker:
+    - image: cimg/base:stable
+    steps:
+      - setup-node
+      - run: npm run lint
+
   Spell check:
     docker:
     - image: cimg/base:stable
@@ -96,5 +103,6 @@ workflows:
                 - "18"
       - "Check for FIXM\x45"
       - Prettier
+      - ESLint
       - Spell check
       - Smoke test built package

.eslintignore

@@ -0,0 +1,3 @@
+dist
+generated
+smoke-test

.eslintrc.cjs

@@ -0,0 +1,16 @@
+module.exports = {
+  root: true,
+  parser: '@typescript-eslint/parser',
+  plugins: ['@typescript-eslint', 'import'],
+  extends: ['plugin:import/typescript'],
+  overrides: [
+    {
+      // Enable import/extensions on all TS files because our ESM builds require
+      // you to specify local imports as full paths with extensions. We don't
+      // need this on tests because Jest doesn't require it.
+      files: ['**/*.ts'],
+      excludedFiles: '**/__tests__/**/*.ts',
+      rules: { 'import/extensions': ['error', 'ignorePackages'] },
+    },
+  ],
+};