Disable syncing from local if its default after iCloud.add

[hank121314]

Description

This PR fixes #172.

By default, Defaults will create a sync task that automatically detects the data source using timestamps. However, this might cause an issue if the device is newly installed, as it might accidentally push the default value to iCloud. It's also unnecessary to publish the default value to iCloud upon initialization, since all devices should already have that value.

Implementation

Added a new function isDefaultValue for Defaults.Key<Value>.
To detect if the locally stored value is a default value, I had to change Defaults.iCloud.add to use a generic packed parameter.

Question

This change introduces a new issue: we cannot support Defaults.iCloud.add with an array of Defaults.Key<Value> due to the lack of variadic parameter support. Is there any way to resolve this?

[sindresorhus]

This should be a property and I don't think it should be public as it wouldn't work for non-equatable values.

We can instead add a version of this publicly:

Defaults/Sources/Defaults/SwiftUI.swift

Lines 162 to 167 in bf71746

	extension Default where Value: Equatable {
	/**
	Indicates whether the value is the same as the default value.
	*/
	public var isDefaultValue: Bool { wrappedValue == defaultValue }
	}

[sindresorhus]

Untested and would require Swift 6, but maybe:

public static func add<each Value: Defaults.Serializable>(_ keys: repeat [Defaults.Key<each Value>]) {
	for key in repeat each keys {
		add(key)
	}
}

[hank121314]

Yeah, will add a TODO for it first!

[sindresorhus]

Looks great. Thanks for fixing this 👍