tools: lint doc code examples in strict mode

PR-URL: #21615
Refs: https://github.com/eslint/eslint-plugin-markdown#strict
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

.eslintrc.js

@@ -40,6 +40,11 @@ module.exports = {
       ],
       parserOptions: { sourceType: 'module' },
     },
+    {
+      files: ['**/*.md'],
+      parserOptions: { ecmaFeatures: { impliedStrict: true } },
+      rules: { strict: 'off' },
+    },
   ],
   rules: {
     // ESLint built-in rules

doc/.eslintrc.yaml

@@ -5,7 +5,6 @@ rules:
   no-restricted-properties: off
   no-undef: off
   no-unused-vars: off
-  strict: off
   symbol-description: off
 
   # add new ECMAScript features gradually