Add codec generation for simple opaque types

[plokhotnyuk]

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[Copilot]

Pull Request Overview

This PR adds codec generation support for Scala 3 opaque types in the jsoniter-scala-macros library. The changes enable automatic JSON serialization and deserialization of opaque types by treating them as their underlying types.

• Adds codec generation logic for opaque types that delegates to the underlying type's codec
• Introduces test cases demonstrating opaque type serialization/deserialization
• Updates terminology from "opaque types" to "newtypes" in existing test descriptions

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
JsonCodecMaker.scala	Core implementation adding opaque type support in read/write/key codec generation
JsonCodecMakerNewTypeSpec.scala	Test cases for opaque types and updates to existing Year type usage
JsonCodecMakerSpec.scala	Minor terminology update in test description

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