Fix JSONEncoder.OutputFormatting.sortedKeys behavior

[woxtu]

This PR tries to fix JSONEncoder.OutputFormatting.sortedKeys behavior. Resolves #4702.

I estimated string comparison options by trying the following code on macOS.

let testValue = [
    "2": "2",
    "25": "25",
    "7": "7",
    "ａｌｉｃｅ": "ａｌｉｃｅ",
    "bob": "bob",
    "Charlie": "Charlie",
    "日本": "日本",
    "中国": "中国",
    "韓国": "韓国",
]

let encoder = JSONEncoder()
encoder.outputFormatting = .sortedKeys

let data = try! encoder.encode(testValue)
let string = String(data: data, encoding: .utf8)!
print(string)
// {"2":"2","7":"7","25":"25","ａｌｉｃｅ":"ａｌｉｃｅ","bob":"bob","Charlie":"Charlie","中国":"中国","日本":"日本","韓国":"韓国"}

print(testValue.sorted { $0.key.compare($1.key, options: [.caseInsensitive, .diacriticInsensitive, .forcedOrdering, .numeric, .widthInsensitive]) == .orderedAscending }.map(\.key))
// ["2", "7", "25", "ａｌｉｃｅ", "bob", "Charlie", "中国", "日本", "韓国"]

[YOCKOW]

@swift-ci Please test