Ensure vscode extension considers github-markdownlint config

Author: sampart

.vscode/settings.json

@@ -47,15 +47,21 @@
   // but markdownlint only considers that config when linting files in
   // extensions/ql-vscode/ or its subfolders. Therefore, we have to explicitly
   // load the custom rules here too.
-  //
-  // Configuration of the default rules is done in .markdownlint.json, which
-  // is picked up by the extension automatically, and read explicitly in
-  // .markdownlint-cli2.cjs
   "markdownlint.customRules": [
     "./extensions/ql-vscode/node_modules/@github/markdownlint-github/src/rules/no-default-alt-text.js",
     "./extensions/ql-vscode/node_modules/@github/markdownlint-github/src/rules/no-generic-link-text.js"
   ],
 
+  // This ensures that the rule enablement done by github-markdownlint is considered
+  // by the extension too.
+  //
+  // Additional nonfiguration of the default rules is done in .markdownlint.json,
+  // which is picked up by the extension automatically, and read explicitly in
+  // .markdownlint-cli2.cjs
+  "markdownlint.config": {
+    "extends": "./extensions/ql-vscode/node_modules/@github/markdownlint-github/style/accessibility.json"
+  },
+
   // These options are used by the `jestrunner.debug` command.
   // They are not used by the `jestrunner.run` command.
   // After clicking "debug" over a test, continually invoke the