content: Follow web in setting 1.4x line height for code blocks

(And apply the same change to our simplified math blocks, which use
this variable too.)

This was ~1.29x in the previous commit, and 1.43x before that. (The
1.43x was conveniently very close to 1.4x, but that was basically an
accident. It's so close that on my device I can't see a difference
in a screenful of code block content...perhaps there's a step that
rounds to a physical-pixel count, and the difference might be
observable on a device with a higher pixel density than mine.)

Author: chrisbobbe

lib/widgets/content.dart

@@ -858,7 +858,10 @@ final _kInlineCodeStyle = kMonospaceTextStyle
     backgroundColor: const HSLColor.fromAHSL(0.04, 0, 0, 0).toColor()));
 
 final _kCodeBlockStyle = kMonospaceTextStyle
-  .merge(const TextStyle(fontSize: 0.825 * kBaseFontSize));
+  .merge(const TextStyle(
+    fontSize: 0.825 * kBaseFontSize,
+    height: 1.4,
+  ));
 
 // const _kInlineCodeLeftBracket = '⸤';
 // const _kInlineCodeRightBracket = '⸣';