Suggested Feature: Designer Support for Missing Event Handlers

### Background and motivation

Currently, when you have a control, say a button, in the visual form designer, and you double-click it, it generates a normal method in the the MyForm.cs file, such as:
```
private void button_Click(object sender, EventArgs e)
{

}
```
And in MyForm.designer.cs it just adds
```
this.button.Clicked += new EventHanlder(this.button_Click);
```
Now, if you will remove this method, it will cause a designer error, and you'll have to go to the designer code and manually delete this line. For beginners it's especially problematic...

I think that a better approach would be to generate partial method, and add its signature in the designer:
```
this.button.Clicked += new EventHanlder(this.button_Click);
...
partial void button_Click(object sender, EventArgs e);
```
This way, if you will delete the method implementation, it won't be an error.

### API Proposal

```csharp
partial class Form1 : Form
{
private partial void Button1_Click(object sender, EventArgs e)
{

}
}
```

### API Usage

Form1.designer.cs:

```csharp
private void InitializeComponent()
{
...
this.Button1.Clicked += new System.EventHandler(this.Button1_Click);
}

private Button Button1;
private partial void Button1_Click(object sender, EventArgs e);
```

### Alternative Designs

_No response_

### Risks

_No response_

### Will this feature affect UI controls?

The VS designer will need to generate event methods as partial methods.