content: Support basic text styles for KaTeX content

This adds another experimental flag called `forceRenderKatex`
which, if enabled, ignores any errors generated by the parser
(like when encountering an unsupported CSS class) tries to do
a "broken" render of the available span and their styles.
Allowing the developer to test the different KaTeX content in
the wild easily, while still in development.

Author: rajveermalviya

lib/model/content.dart

@@ -371,11 +371,14 @@ class MathBlockNode extends BlockContentNode {
 
 class KatexNode extends ContentNode {
   const KatexNode({
+    required this.styles,
     required this.text,
     required this.nodes,
     super.debugHtmlNode,
   }) : assert((text != null) ^ (nodes != null));
 
+  final KatexSpanStyles styles;
+
   /// The text this KaTeX node contains.
   ///
   /// It will be null if [nodes] is non-null.
@@ -389,6 +392,7 @@ class KatexNode extends ContentNode {
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
+    properties.add(DiagnosticsProperty<KatexSpanStyles>('styles', styles));
     properties.add(StringProperty('text', text));
   }