Prettier marshalling of a11y data

In the output, we can get for example:
```json
"accessibility": {
  "conformsTo": ["https://www.w3.org/TR/epub-a11y-11#wcag-2.2-aa"],
  "accessModeSufficient": [["textual"]],
}
```
These properties could be better marshalled and result in the following output:
```json
"accessibility": {
  "conformsTo": "https://www.w3.org/TR/epub-a11y-11#wcag-2.2-aa",
  "accessModeSufficient": ["textual"],
}
```