fix(eslint): move to optional peer dependencies

Resolves #16

.eslintrc.js

@@ -1,5 +1,5 @@
 module.exports = {
-  extends: './eslint/node',
+  extends: ['./eslint/node'],
   overrides: [
     {
       files: ['eslint/rules/**'],

eslint/next.js

@@ -1,4 +1,8 @@
 const { JAVASCRIPT_FILES } = require('./constants');
+const requirePackage = require('./utils/require-package');
+
+requirePackage('next', 'next');
+requirePackage('next', '@next/eslint-plugin-next');
 
 const babelOptions = {
   presets: (() => {

eslint/typescript.js

@@ -1,4 +1,7 @@
 const { TYPESCRIPT_FILES } = require('./constants');
+const requirePackage = require('./utils/require-package');
+
+requirePackage('typescript', 'typescript');
 
 module.exports = {
   overrides: [

eslint/utils/require-package.js

@@ -0,0 +1,36 @@
+// @ts-check
+const pkgJson = require('../../package.json');
+
+/**
+ * @returns {string} The name of the package manager.
+ */
+function readPackageManager() {
+  const match = process.env.npm_config_user_agent?.match(/^(?<pm>\w+)\//);
+  return match?.groups ? match.groups?.pm : 'npm';
+}
+
+/**
+ * @param {string} configName
+ * @param {string} packageName
+ */
+module.exports = (configName, packageName) => {
+  try {
+    require.resolve(packageName);
+  } catch (e) {
+    const packageManager = readPackageManager();
+    const command = packageManager === 'yarn' ? 'add' : 'install';
+
+    /* eslint-disable no-console */
+    console.error(
+      `The \`${configName}\` config requires an optional peer dependency, which has not been installed.`,
+    );
+    console.error();
+    console.error('To install it, run:');
+    console.error(
+      `- ${packageManager} ${command} ${packageName}@${pkgJson.peerDependencies[packageName]}`,
+    );
+    /* eslint-enable no-console */
+
+    process.exit(1);
+  }
+};

package.json

@@ -26,6 +26,7 @@
   },
   "prettier": "./prettier",
   "dependencies": {
+    "@babel/core": "^7.19.1",
     "@babel/eslint-parser": "^7.19.1",
     "@rushstack/eslint-patch": "^1.2.0",
     "@typescript-eslint/eslint-plugin": "^5.38.0",
@@ -46,7 +47,6 @@
     "prettier-plugin-packagejson": "^2.2.18"
   },
   "devDependencies": {
-    "@babel/core": "^7.19.1",
     "@commitlint/cli": "^17.1.2",
     "@commitlint/config-conventional": "^17.1.0",
     "@semantic-release/git": "^10.0.1",
@@ -57,17 +57,13 @@
     "typescript": "^4.8.3"
   },
   "peerDependencies": {
-    "@babel/core": "^7.19.1",
     "@next/eslint-plugin-next": "^12.3.0",
     "eslint": "^8.12.0",
     "next": "^12.3.0",
     "prettier": "^2.6.1",
     "typescript": "^4.8.3"
   },
   "peerDependenciesMeta": {
-    "@babel/core": {
-      "optional": true
-    },
     "@next/eslint-plugin-next": {
       "optional": true
     },