Expose Control.IsFontSet publicly

### Background and motivation

There is an internal method `Control.IsFontSet()` which determines if the `Font` property has been explicitly set on the control. This is as opposed to the `Font` property returning the font of the parent control or `Control.DefaultFont`.

When dealing with scaling it is very useful to know if the `Font` has been explicitly set. This allows you to avoid scaling the font if it's unnecessary because the parent font is already scaled. Internally this is done by `Control` for DPI scaling, however we have scenarios where we want to scale based on other factors. We're currently using reflection to access `Control.IsFontSet()` to get the performance benefit, but this is clearly risky long-term and less performant due to reflection dispatch, boxing, lack of inlining, etc.

### API Proposal

```diff
namespace System.Windows.Forms;

public partial class Control
{
+ public bool IsFontSet { get; }
}
```

### API Usage

```csharp
private static void ScaleFonts(Control ctrl, float factor)
{
if (ctrl.HasChildren)
{
foreach (Control childCtrl in ctrl.Controls)
{
ScaleFonts(childCtrl, factor);
}
}

if (ctrl.IsFontSet)
{
var curFont = ctrl.Font;
ctrl.Font = new Font(curFont.FontFamily, curFont.Size * factor, curFont.Style, curFont.Unit,
curFont.GdiCharSet, curFont.GdiVerticalFont);
}
}
```

### Alternative Designs

An alternative design could be to leave `IsFontSet` as a method instead of a read-only property. Also, it could be made virtual.

Another possibility would be to expose font scaling APIs publicly, such as moving some of the logic in `Control.WmDpiChangedBeforeParent` into a `public void ScaleFont(float scaleFactor)` method. However, exposing `IsFontSet` is more generally useful beyond font scaling.

### Risks

This design has the risk that a descendent `Control` could completely override `Font` behaviors, making `IsFontSet` incorrect.

### Will this feature affect UI controls?

Yes, however, it should have no impact on VS design, accessibility, or localization.