Add support for the font tag

Author: tneotia

lib/src/styled_element.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_html/src/css_parser.dart';
 import 'package:flutter_html/style.dart';
 import 'package:html/dom.dart' as dom;
 //TODO(Sub6Resources): don't use the internal code of the html package as it may change unexpectedly.
@@ -179,6 +180,17 @@ StyledElement parseStyledElement(
         display: Display.BLOCK,
       );
       break;
+    case "font":
+      styledElement.style = Style(
+        color: element.attributes['color'] != null ?
+          element.attributes['color']!.startsWith("#") ?
+            ExpressionMapping.stringToColor(element.attributes['color']!) :
+            ExpressionMapping.namedColorToColor(element.attributes['color']!) :
+          null,
+        fontFamily: element.attributes['face']?.split(",").first,
+        fontSize: numberToFontSize(element.attributes['size'] ?? ''),
+      );
+      break;
     case "h1":
       styledElement.style = Style(
         fontSize: FontSize.xxLarge,
@@ -368,3 +380,31 @@ StyledElement parseStyledElement(
 }
 
 typedef ListCharacter = String Function(int i);
+
+FontSize numberToFontSize(String num) {
+  switch (num) {
+    case "1":
+      return FontSize.xxSmall;
+    case "2":
+      return FontSize.xSmall;
+    case "3":
+      return FontSize.small;
+    case "4":
+      return FontSize.medium;
+    case "5":
+      return FontSize.large;
+    case "6":
+      return FontSize.xLarge;
+    case "7":
+      return FontSize.xxLarge;
+  }
+  if (num.startsWith("+")) {
+    final relativeNum = double.tryParse(num.substring(1)) ?? 0;
+    return numberToFontSize((3 + relativeNum).toString());
+  }
+  if (num.startsWith("-")) {
+    final relativeNum = double.tryParse(num.substring(1)) ?? 0;
+    return numberToFontSize((3 - relativeNum).toString());
+  }
+  return FontSize.medium;
+}