feat: implement Scala 3 Constant and Union support for string-based literals as enums

[ThijsBroersen]

This PR implements support for string-based literal types. Single-value or Union-based.

Example:

case class Foo(aOrB: "A" | "B", optA: Option["A"]) derives Schema

[adamw]

Thanks for the PR! I think for completeness it would also be great to have Codec-deriving macros (if these enumerations are used at the top-level, e.g. for query parameters)

[ThijsBroersen]

@adamw thanks for the feedback, will process it probably tonight.

[ThijsBroersen]

Added both Codec and Validator derivations. Schema and Codec reuse the Validator now.

[adamw]

Thanks!