Fix UserDefaults.string(forKey:) behavior

[woxtu]

This PR fixes UserDefaults.string(forKey:) behavior for number values. Resolves #3362.
https://developer.apple.com/documentation/foundation/userdefaults/1416700-string

UserDefaults.standard.set(true, forKey: "key1")
UserDefaults.standard.set(42, forKey: "key2")
UserDefaults.standard.set(12.34 as Float, forKey: "key3")
UserDefaults.standard.set(12.34, forKey: "key4")
UserDefaults.standard.set("1234", forKey: "key5")

// on macOS
UserDefaults.standard.string(forKey: "key1") // "1"
UserDefaults.standard.string(forKey: "key2") // "42"
UserDefaults.standard.string(forKey: "key3") // "12.34"
UserDefaults.standard.string(forKey: "key4") // "12.34"
UserDefaults.standard.string(forKey: "key5") // "1234"

// on Linux
UserDefaults.standard.string(forKey: "key1") // nil
UserDefaults.standard.string(forKey: "key2") // nil
UserDefaults.standard.string(forKey: "key3") // nil
UserDefaults.standard.string(forKey: "key4") // nil
UserDefaults.standard.string(forKey: "key5") // "1234"

[compnerd]

@swift-ci please test

[woxtu]

Thanks!