Fix caret positioning with empty newlines.

`FormattedTextImpl` on Skia adds a special empty line when there is a trailing newline in the string. However this line is reported as having a width due to `SKPaint.MeasureText` returning a non-zero value for `\r` and `\n`.

We can't simply always use 0 width when we encounter a control char when building the rects as that breaks hit testing. Instead, mark the empty trailing line with a flag and and treat it differently when building the rects and hit-testing a range.

Yes, this is a big hack but as far as I understand the whole of `FormattedTextImpl` on Skia is a bit of a hack, but until @Gillibald's new `TextPresenter` is ready, this _might_ be a good enough fix?

src/Skia/Avalonia.Skia/FormattedTextImpl.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
 using Avalonia.Media;
 using Avalonia.Platform;
@@ -175,7 +176,8 @@ namespace Avalonia.Skia
 
             foreach (var line in _skiaLines.Where(l =>
                                                     (l.Start + l.Length) > index &&
-                                                    lastIndex >= l.Start))
+                                                    lastIndex >= l.Start &&
+                                                    !l.IsEmptyTrailingLine))
             {
                 int lineEndIndex = line.Start + (line.Length > 0 ? line.Length - 1 : 0);
 
@@ -466,7 +468,8 @@ namespace Avalonia.Skia
 
                 for (int i = line.Start; i < line.Start + line.TextLength; i++)
                 {
-                    float w = _paint.MeasureText(Text[i].ToString());
+                    var c = Text[i];
+                    var w = line.IsEmptyTrailingLine ? 0 :_paint.MeasureText(Text[i].ToString());
 
                     _rects.Add(new Rect(
                         prevRight,
@@ -611,6 +614,7 @@ namespace Avalonia.Skia
                         lastLine.Width = lastLineWidth;
                         lastLine.Height = _lineHeight;
                         lastLine.Top = curY;
+                        lastLine.IsEmptyTrailingLine = true;
 
                         _skiaLines.Add(lastLine);
 
@@ -713,6 +717,7 @@ namespace Avalonia.Skia
             public int TextLength;
             public float Top;
             public float Width;
+            public bool IsEmptyTrailingLine;
         };
 
         private struct FBrushRange