expandTypeDepFree and expandTypeBabelTS don't handle JSDocNullableType

[kungfooman]

Working

expandType('number?')

Expected output:

{
  "type": "union",
  "members": [
    "number",
    "null"
  ]
}

Current outputs:

Since Babel has no support for JSDocNullableType right now and it doesn't look like they are willing to even accept a PR, it probably makes most sense to simply do type.endsWith('?') and turn it into a nullable/union-with-null: babel/babel#16073

Same for expandTypeDepFree.