content [nfc]: Name CodeBlockTextStyles's factory constructor "light"

We're about to add a "dark", for #749 / #95 dark theme.

Author: chrisbobbe

lib/widgets/code_block.dart

@@ -9,7 +9,7 @@ import 'text.dart';
 /// Use [forSpan] for syntax highlighting.
 // TODO(#749) follow web for dark-theme colors
 class CodeBlockTextStyles {
-  factory CodeBlockTextStyles(BuildContext context) {
+  factory CodeBlockTextStyles.light(BuildContext context) {
     final bold = weightVariableTextStyle(context, wght: 700);
     return CodeBlockTextStyles._(
       plain: kMonospaceTextStyle

lib/widgets/content.dart

@@ -48,7 +48,7 @@ class ContentTheme extends ThemeExtension<ContentTheme> {
     )
       .merge(weightVariableTextStyle(context))
       .copyWith(debugLabel: 'ContentTheme.textStylePlainParagraph'),
-    codeBlockTextStyles = CodeBlockTextStyles(context),
+    codeBlockTextStyles = CodeBlockTextStyles.light(context),
     textStyleError = const TextStyle(fontSize: kBaseFontSize, color: Colors.red)
       .merge(weightVariableTextStyle(context, wght: 700)),
     textStyleErrorCode = kMonospaceTextStyle