Support extracting a struct to it's own type

[dlsniper]

From the following code:

package main

func main() {
    demo := struct {
        FieldA string
    }{
        FieldA: "demo",
    }
    _ = demo
}

the struct should be extracted to:

package main

type StructName struct { // StructName should be editable before extraction is finished
    FieldA string
}

func main() {
    demo := StructName{
        FieldA: "demo",
    }
    _ = demo
}