[AppKitBackend] Implement foregroundColor modifier

[stackotter]

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We'll need to figure out how to cascade styles through the AppKit view hierarchy. Similarly to what Lorenzo faced with WinUIBackend, this seems to be a little tricky since it's not really how the framework is intended to be used.

[stackotter]

This will likely be best addressed by allowing modifiers to be propagated down the view graph and given to all backend methods so that modifiers like foregroundColor can be applied directly to leaf widgets instead of relying on the underlying UI frameworks having cascading styles.