TextField crashes app when pasting unicode surrogate pair

[tznind]

Describe the bug
When pasting a 2 char pair e.g. 📄 the TextField crashes with following exception:

System.ArgumentOutOfRangeException
Specified argument was out of the range of valid values. (Parameter 'value')
   at System.Text.Rune..ctor(UInt32 value)
   at Terminal.Gui.Key.ToRune(KeyCode key) in D:\Repos\temp\gui.cs\Terminal.Gui\Input\Keyboard\Key.cs:line 349
   at Terminal.Gui.Key.get_AsRune() in D:\Repos\temp\gui.cs\Terminal.Gui\Input\Keyboard\Key.cs:line 156
   at Terminal.Gui.TextField.InsertText(Key a, Boolean usePreTextChangedCursorPos) in D:\Repos\temp\gui.cs\Terminal.Gui\Views\TextField.cs:line 1389
   at Terminal.Gui.TextField.InsertText(String toAdd, Boolean useOldCursorPos) in D:\Repos\temp\gui.cs\Terminal.Gui\Views\TextField.cs:line 750
   at Terminal.Gui.ViewsTests.TextFieldTests.TestPasteUnicodeTextField() in D:\Repos\temp\gui.cs\Tests\UnitTests\Views\TextFieldTests.cs:line 1982
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

To Reproduce
Steps to reproduce the behavior:

    [Fact]
    public void TestPasteUnicodeTextField ()
    {
        var tf = new TextField ();
        //📄
        tf.InsertText ("\ud83d\udcc4");

    }

Expected behavior
If it is not supported by Terminal.Gui then ignore the paste i.e. paste nothing

[BDisp]

TextField doesn't use the InsertText method to paste from the clipboard but use the Paste method setting the Text property directly, beside the TextField displaying the replacement character. But the InsertText method should also be able to process it. So, it needed to process as rune and not as char.

[tig]

Thanks. I just realized i had not pulled down the latest v2_develop.

I'll fix it all in #4046.