[BUG]: Generating Scala3 uPickle from JSON Schema is missing ReadWriter annotation for enums

## Issue Type

Output

## Context (Environment, Version, Language)

Input Format: JSON Schema
Output Language: Scala3+uPickle

CLI, npm, or app.quicktype.io: CLI
Version: 23.0.170

## Description

In the generated Scala3 output there is a missing `ReadWriter` annotation for the generated `enum Color`, which leads to a build failure while the uPickle macro is trying run:
```
No given instance of type ReadersVersionSpecific_this.Reader[
[error] | quicktype.Color] was found.
```

## Input Data

```json
{
"$schema": "https://json-schema.org/draft/2019-09/schema",
"definitions": {
"Color" : {
"type" : "string",
"enum": ["Red", "Green", "Blue"]
},
"Monster" : {
"type" : "object",
"properties" : {
"name" : {
"type" : "string"
},
"color" : {
"$ref" : "#/definitions/Color"
}
},
"required" : ["name", "color"],
"additionalProperties" : false
}
},
"$ref" : "#/definitions/Monster"
}
```

## Expected Behaviour / Output

```scala
package quicktype

import upickle.default.*

case class Monster (
val color : Color,
val name : String
) derives ReadWriter

enum Color derives ReadWriter :
case Blue,Green,Red
```

## Current Behaviour / Output

```scala
package quicktype

import upickle.default.*

case class Monster (
val color : Color,
val name : String
) derives ReadWriter

enum Color :
case Blue,Green,Red

```

## Steps to Reproduce

It also reproduces in the webapp, so easiest repro is:

1. paste above json into webapp and JSON Schema as source
2. Select Scala3 and uPickle as target
3. Try to compile the resulting code with with a minimal sbt project. Something along the lines of:
```scala
val scala3Version = "3.6.2"

lazy val root = project
.in(file("."))
.settings(
name := "Quicktype Test",
version := "0.1.0-SNAPSHOT",

scalaVersion := scala3Version,
libraryDependencies += "com.lihaoyi" %%% "upickle" % "4.1.0"
)
```

## Possible Solution

Just adjust the generator to add the necessary derive for enums.