Add the `partial` modifier to the generated property of the Relay Command

### Overview

Add the **partial** modifier to the generated property of the Relay Command.

Now, app models like .NET MAUI have introduced a source generation feature that allows XAML to be translated into C#. However, the issue remains that source generators cannot interact with one another, such that only handwritten code is accessible.

Since the Observable Property is partial, it can be referenced in XAML without any issues. However, the Relay Command property is not partial, which prevents XAML source generation from inferring it. To resolve this, update the generated Relay Command property partial so that its declaration can be added in the handwritten code, while the MVVM toolkit handles the implementation.

### API breakdown

Providing the source-generated code for reference. The generated property needs to be **partial**.

**User code:**

```cs
public partial IRelayCommand IncrementCommand { get; }
```

**Generated:**

```cs
private global::CommunityToolkit.Mvvm.Input.RelayCommand? incrementCommand;
// The request is make this IncrementCommand property partial
// so that the developer can add a declaration part in the source type.
public global::CommunityToolkit.Mvvm.Input.IRelayCommand IncrementCommand => incrementCommand ??= new global::CommunityToolkit.Mvvm.Input.RelayCommand(new global::System.Action(Increment));
```

### Usage example

The `IncrementCommand` is now visible to the App Model source generator.

Previously, the evaluation occurred at runtime; now, to support AOT, it is resolved at compile time. To avoid reflection, the C# expression API is used, requiring at least the declaration/definition to be part of the handwritten code.

```xml

```

### Breaking change?

I'm not sure

### Alternatives

Manually create the command property, losing all the toolkit's benefits.

### Additional context

Multiple source generators at work.

### Help us help you

No, just wanted to propose this