content: Move CodeBlock padding to inside scroll view

This way, when the text is wide and needs to scroll, the padding
scrolls with the text, rather than reducing the size of the viewport.
See screenshots:
  #242 (comment)

Author: rajveermalviya

lib/widgets/content.dart

@@ -257,7 +257,6 @@ class CodeBlock extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return Container(
-      padding: const EdgeInsets.fromLTRB(7, 5, 7, 3),
       decoration: BoxDecoration(
         color: Colors.white,
         border: Border.all(
@@ -266,7 +265,9 @@ class CodeBlock extends StatelessWidget {
         borderRadius: BorderRadius.circular(4)),
       child: SingleChildScrollViewWithScrollbar(
         scrollDirection: Axis.horizontal,
-        child: Text.rich(_buildNodes(node.spans))));
+        child: Padding(
+          padding: const EdgeInsets.fromLTRB(7, 5, 7, 3),
+          child: Text.rich(_buildNodes(node.spans)))));
   }
 
   InlineSpan _buildNodes(List<CodeBlockSpanNode> nodes) {