content: Use weightVariableTextStyle in ContentTheme.textStyleError

This converts the last remaining place where we were using
TextStyle.fontWeight directly, when we should have been using
weightVariableTextStyle.

(In a future with
  flutter/flutter#148026
, though, we'll want to change all uses of weightVariableTextStyle
back to plain TextStyle.fontWeight.)

Now, this text is styled with a `wght` value of 700, instead of 400
from the ambient style. The FontWeight.bold seems to have already
been causing the spans to render more boldly than surrounding text,
but in a way that doesn't match what we get for Source Code Pro with
`wght` 700. I think this is because of
  flutter/flutter#136779
.

Author: chrisbobbe

lib/widgets/content.dart

@@ -47,8 +47,8 @@ class ContentTheme extends ThemeExtension<ContentTheme> {
     )
       .merge(weightVariableTextStyle(context))
       .copyWith(debugLabel: 'ContentTheme.textStylePlainParagraph'),
-    textStyleError = const TextStyle(
-      fontSize: kBaseFontSize, fontWeight: FontWeight.bold, color: Colors.red),
+    textStyleError = const TextStyle(fontSize: kBaseFontSize, color: Colors.red)
+      .merge(weightVariableTextStyle(context, wght: 700)),
     textStyleErrorCode = kMonospaceTextStyle
       .merge(const TextStyle(fontSize: kBaseFontSize, color: Colors.red));