Embed the notebook kernel information in page metadata

[choldgraf]

Right now when we parse notebooks, we throw away any notebook-level metadata that is not string/int/float:

https://github.com/ExecutableBookProject/MyST-NB/blob/master/myst_nb/parser.py#L173

However, this throws away the notebook kernel information, which may be useful for interactive tools like Thebelab. Here's the structure it has:

 "metadata": {
   "kernelspec": {
     "display_name": "Python 3",
     "language": "python",
     "name": "python3"
   }
  }

Can we either:

1. Special-case the kernel information and put it in the metadata of the page (really we just need name, but perhaps language as well.
2. Keep non int/float/string values as strings in the page metadata? (that's what we currently do for manually-specified Front Matter in myst-markdown)

[chrisjsewell]

Yeh just json serialize

[choldgraf]

This is fixed in #147