Add storybook/addon-a11y package

Author: koesie10

extensions/ql-vscode/.storybook/main.ts

@@ -6,6 +6,7 @@ const config: StorybookConfig = {
     "@storybook/addon-links",
     "@storybook/addon-essentials",
     "@storybook/addon-interactions",
+    "@storybook/addon-a11y",
     "./vscode-theme-addon/preset.ts",
   ],
   framework: {