If `StructTypes.subtypes` has length one, then we shouldn't need a subtype key

[Dilum Aluthge]

If we have an abstract type, and we have only defined one concrete subtype (i.e. StructTypes.subtypes has length one), then JSON3 should automatically pick this subtype, whether or not StructTypes.subtypekey has been defined.

Here is an example. We have Vehicle, which we declare as StructTypes.StructType(::Type{Vehicle}) = StructTypes.AbstractType(). Then we only declare a single subtype: StructTypes.subtypes(::Type{Vehicle}) = (Car=Car,). So JSON3 should always read Vehicles in as Cars.

julia> import JSON3, StructTypes

julia> abstract type Vehicle end

julia> mutable struct Car <: Vehicle
           topSpeed::Float64
           model::String
           make::String
           seatingCapacity::Int
           Car() = new()
       end

julia> StructTypes.StructType(::Type{Vehicle}) = StructTypes.AbstractType()

julia> StructTypes.StructType(::Type{Car}) = StructTypes.Mutable()

julia> StructTypes.subtypes(::Type{Vehicle}) = (Car=Car,)

julia> j = """
       {
           "make": "Mercedes-Benz",
           "model": "S500",
           "seatingCapacity": 5,
           "topSpeed": 250.1
       }
       """
"{\n    \"make\": \"Mercedes-Benz\",\n    \"model\": \"S500\",\n    \"seatingCapacity\": 5,\n    \"topSpeed\": 250.1\n}\n"

julia> car = JSON3.read(j, Vehicle)
ERROR: ArgumentError: invalid json abstract type: didn't find subtypekey
Stacktrace:
 [1] #read#41
   @ ~/.julia/packages/JSON3/LtYDK/src/structs.jl:617 [inlined]
 [2] read
   @ ~/.julia/packages/JSON3/LtYDK/src/structs.jl:555 [inlined]
 [3] read(str::String, ::Type{Vehicle}; kw::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ JSON3 ~/.julia/packages/JSON3/LtYDK/src/structs.jl:34
 [4] read(str::String, ::Type{Vehicle})
   @ JSON3 ~/.julia/packages/JSON3/LtYDK/src/structs.jl:33
 [5] top-level scope
   @ REPL[8]:1

[Dilum Aluthge]

Fixed by #97