Generate public readonly properties instead of private fields

[Eastrall]

At this moment, Rosalina generates private fields for that represents the named UI components defined in the UXML template.
But if we want to interect with properties within another document, a public field or property is required. Thus, I believe the use of public readonly property could be interesting.

public to grant access to the property.
"readonly" to prevent the user to reassign the inner UI field.

This would give us something like:

// MyDocument.g.cs
public partial class MyDocument
{
    // Root property has been omitted
    public Button MyButton { get; private set; }

    public void InitializeDocument()
    {
        MyButton = (UnityEngine.UIElements.Button)Root?.Q("MyButton");
    }
}

In the extension we will be able to use the property.

public partial class MyDocument
{
     private void OnEnable()
     {
         MyButton.clicked += OnClicked;
     }

    private void OnClicked()
    {
        // Do something
    }
}

When using a MyDocument instance, you'll be able to calls the MyButton property.

// other component
public class OtherComponent : MonoBehavior
{
    private void OnEnable()
    {
        var myDocument = new MyDocument();
       myDocument.MyButton.text = "Hello world!"; // OK !
       myDocument.MyButton = new UnityEngine.UIElements.Button(); // KO !
    } 
}