Change keypath BindingTarget.init to solve source breaking change in SE-0481

[mluisbrown]

Fixes: #889

SE-0481 would break the current version of the BindingTarget.init that takes a keypath. Changing the keypath type from WritableKeyPath to ReferenceWritableKeyPath fixes this, and is probably what the type should always have been, since Object will always be a reference type.

Despite the signature of the initalizer changing, this is unlikely to break existing code using it, as the keypath is almost always passed using keypath notation (eg : \.property) and not using an explicit keypath type.

Checklist

• Updated CHANGELOG.md.

[Copilot]

Pull Request Overview

This PR fixes a source-breaking issue with SE-0481 by changing the key path type in the BindingTarget initializer.

• Updated the initializer to use ReferenceWritableKeyPath instead of WritableKeyPath.
• The change ensures that the key path parameter is appropriate for object reference types.

Comments suppressed due to low confidence (1)

Sources/UnidirectionalBinding.swift:188

• Update the documentation comment to indicate that the keyPath parameter now expects a ReferenceWritableKeyPath to match the new API signature.

public init<Object: AnyObject>(on scheduler: Scheduler = ImmediateScheduler(), lifetime: Lifetime, object: Object, keyPath: ReferenceWritableKeyPath<Object, Value>) {