fix issue with Xcode 16 format and shellScript

[michaelversus]

Resolves #921

Short description 📝

This MR fixes an issue with projects that use the new format, where for build phases, a shell script is an array of strings instead of a string.

Solution 📦

I am deserializing the script with a fallback approach.
If we fail deserializing using the String type we are trying the [String] type to ensure that I will not break old xcode version format

Implementation 👩‍💻👨‍💻

• Step 1 (Introduced Either Decodable helper wrapper)
• Step 2 (Migrate shellScript type from String? to Either<String, [String]>?)
• Step 3 (Removed unused testDictionary function from PBXShellScriptBuildPhaseTests)
• Step 3 (Introduce new fixture and update unit tests)

[michaelversus]

@fortmarek I would really appreciate your feedback on this.