generator: Sort keys in JSON metadata files (#171)

This change makes it possible to compare generated SDKs using
recursive diff, without having to account for spurious differences
cause by the order of keys in JSON metadata files.

Author: euanh

Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator+Metadata.swift

@@ -16,7 +16,7 @@ import class Foundation.JSONEncoder
 
 private let encoder: JSONEncoder = {
   let encoder = JSONEncoder()
-  encoder.outputFormatting = [.prettyPrinted, .withoutEscapingSlashes]
+  encoder.outputFormatting = [.prettyPrinted, .withoutEscapingSlashes, .sortedKeys]
   return encoder
 }()