[API Proposal]: ProcessStartInfo: enable killing process on dispose

### Background and motivation

`Process.Dispose` only releases associated resources (handles) without terminating the child process. This frequently leads to leaked child processes when developers use the natural `using` pattern:

```csharp
using Process process = Process.Start("long-running-tool")!;
// ... use the process ...
// process.Dispose() is called here, but the child process keeps running
```

Today, to ensure a child process is killed on dispose, users must write manual try/finally blocks:

```csharp
using Process process = Process.Start("long-running-tool")!;
try
{
// ... use the process ...
}
catch
{
process.Kill(entireProcessTree: true);
throw;
}
```

A `ProcessStartInfo.DisposeBehavior` property would allow the `using` pattern to work as users intuitively expect and eliminate an entire class of leaked-process bugs.

### API Proposal

```csharp
namespace System.Diagnostics
{
public enum ProcessDisposeBehavior
{
None = 0,
KillProcess = 1,
KillProcessTree = 2
}

public partial class ProcessStartInfo
{
public ProcessDisposeBehavior DisposeBehavior { get; set; }
}
}
```

### API Usage

**Basic usage — kill a long-running process tree when done:**

```csharp
using Process process = Process.Start(new ProcessStartInfo("dev-server")
{
DisposeBehavior = ProcessDisposeBehavior.KillProcessTree
})!;

// ... interact with the dev server ...

// process.Dispose() is called here, which kills the process tree first
```