katex: Render overline and underline with border decoration.

Implements the visual rendering of overline and underline elements
using Flutter's DecoratedBox with a bottom border. When a span has
a solid border-bottom-style and border width, we wrap the widget
in a DecoratedBox with a BorderSide.

The border color matches the katexSpan  color (or defaults to the current text style color), and the border width is calculated from the em value specified in the inline styles.

Author: MdSaifAliMolla

lib/widgets/katex.dart

@@ -125,6 +125,19 @@ class _KatexSpan extends StatelessWidget {
       null => null,
     };
 
+    if (styles.borderBottomStyle == KatexSpanBorderBottomStyle.solid &&
+        styles.borderBottomWidthEm != null) {
+      final borderColor = color ?? DefaultTextStyle.of(context).style.color!;
+      final borderWidth = styles.borderBottomWidthEm! * em;
+
+      widget = DecoratedBox(
+        decoration: BoxDecoration(
+          border: Border(
+            bottom: BorderSide(color: borderColor, width: borderWidth, style: BorderStyle.solid))),
+        child: widget,
+      );
+    }
+
     TextStyle? textStyle;
     if (fontFamily != null ||
         fontSize != null ||