Optional markdown config

Hi! Thanks a lot for the hardworking. I'm currently in the process of refactoring our company's shareable config to the new flat implementation and your repository has been really helpful. While reading the code I noticed that in the `src/presets.ts` file, the `markdown` is imported and then spread inside the `presetLangsExtensions` array. This array is itself spread into the `basic` array, but then you pass a param to your config as such `markdown: enableMarkdown = true`. To my current understanding and knowledge, I believe this is currently having no impact since you're either using basic with the markdown or using basic with the markdown twice.