Add layout option and * fallback of meta.json

[shuding]

Breaking change:

"theme": {
  "full": true
}

will be:

"theme": {
  "layout": "full"
}

It will support three options: "default", "full" and "raw". "raw" will not be wrapped by <MDXContent> and article.

---

Also the * key will be used as the fallback configuration for all pages in the same level:

{
  "*": {
    "type": "page",
    "theme": {
      "sidebar": true
    }
  },
  "a": "A",
  "b": {
    "title": "B",
    "theme": {
      "sidebar": false
    }
  }
}

[vercel]

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