fix: fix build for iOS, tvOS, and watchOS

[kkebo]

Your package supports iOS 12, tvOS 12, and watchOS 5 according to Package.swift.

However, withoutEscapingSlashes requires iOS 13+, tvOS 13+, or watchOS 6+.

https://developer.apple.com/documentation/foundation/jsonserialization/writingoptions/3081260-withoutescapingslashes

So, you have two options.

1. Wrap the code using withoutEscapingSlashes with if #available
2. Bump minimum deployment targets

This pull request will resolve the issue by the option 1.