Provide simpler core types

[mcwalrus]

The current datatype presented is:

type JitJSON[T any] struct {
	data []byte
	val  *T
        parser JSONParser
}

However parser is a redundant type which can lead to inefficiently / increased allocation. I would prefer if the library supported two types JITJSON / JITJSON_V2 for json/encoding v1 and v2 support. README and go docs would need to be updated to only cover the simpler interface.