Description

Recently #3604 added the GraphiQLPluginExplorer

When using the GraphQL Playground handler, if the WithGraphiqlEnablePluginExplorer(true) option is not explicitly passed, the plugin explorer script (@graphiql/plugin-explorer) is not included in the rendered HTML. However, the JavaScript code in the template still references GraphiQLPluginExplorer, leading to the following error in the browser console:

Uncaught ReferenceError: GraphiQLPluginExplorer is not defined
    at <anonymous>:113

Steps to Reproduce

1. Set up the GraphQL Playground handler without enabling the plugin explorer:

   h := playground.Handler("GraphQL Playground", "/graphql/query")

2. Open the playground in the browser.
3. Check the console for the GraphiQLPluginExplorer is not defined error.

Resolves #3619