Add .eslintrc.cjs to list of supported next lint config files (#3…

…5440)

The logic for finding a user's custom `.eslintrc` config file is missing a check for `.eslintrc.cjs`, which is listed as one of the valid filenames in ESLint's docs:

https://eslint.org/docs/user-guide/configuring/configuration-files#configuration-file-formats

...so this PR makes `next lint`'s list of possible config files consistent with ESLint's.

I stumbled upon this when trying to rename my own config file, since I've pretty much moved completely to ESM by now... but [ESLint doesn't support modules yet](eslint/eslint#14137 (comment)), so renaming the file to `.cjs` just keeps things cleaner (and VS Code happier). This is my first PR here so even though I've only changed a couple of lines, please let me know if I'm overlooking something! 😊

## Bug

- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

packages/next/lib/eslint/runLintCheck.ts

@@ -265,10 +265,12 @@ export async function runLintCheck(
 ): ReturnType<typeof lint> {
   try {
     // Find user's .eslintrc file
+    // See: https://eslint.org/docs/user-guide/configuring/configuration-files#configuration-file-formats
     const eslintrcFile =
       (await findUp(
         [
           '.eslintrc.js',
+          '.eslintrc.cjs',
           '.eslintrc.yaml',
           '.eslintrc.yml',
           '.eslintrc.json',

test/integration/eslint/test/index.test.js

@@ -348,6 +348,7 @@ describe('ESLint', () => {
         (await findUp(
           [
             '.eslintrc.js',
+            '.eslintrc.cjs',
             '.eslintrc.yaml',
             '.eslintrc.yml',
             '.eslintrc.json',