support parse json string to generate variables

[wleii]

example:

struct Animal: Codable {
'''
{
"type":"Pet",
"pet": {
	"name":"Doge",
	"gender": "male",
	"age":8
}
'''
}

result:

struct Animal: Codable {
    let type: String
    let pet: Pet
    struct Pet: Codable {
         ...
    }
}