[DTCG] Implement a way to map group types to spec's supported token types

[apresmoi]

As pointed out in #8 we should make the following fixes on the mapping of group types to token types:

Group Type	Token Type	Reason
spacing	dimension
radius	string	Actually this could be dimension, but we also have to support % and currently there isn't support for that in the spec
mediaQuery	dimension
fontSize	dimension
letterSpacing	dimension
lineHeight	dimension

Example:

That should look something like:

{
  "Spacing": {
    "$type": "dimension",
    "tokens": {
      "small": {
        "$value": "4px"
      },
      "medium": {
        "$value": "8px"
      },
    }
  }
}

[apresmoi]

This was solved in #15