chore: update the menu item size for code_block, horizaontal_rule and tex_block

frontend/app_flowy/packages/appflowy_editor/example/lib/plugin/code_block_node_widget.dart

@@ -46,7 +46,11 @@ ShortcutEventHandler _ignorekHandler = (editorState, event) {
 
 SelectionMenuItem codeBlockMenuItem = SelectionMenuItem(
   name: () => 'Code Block',
-  icon: const Icon(Icons.abc),
+  icon: const Icon(
+    Icons.abc,
+    color: Colors.black,
+    size: 18.0,
+  ),
   keywords: ['code block'],
   handler: (editorState, _, __) {
     final selection =

frontend/app_flowy/packages/appflowy_editor/example/lib/plugin/horizontal_rule_node_widget.dart

@@ -38,7 +38,11 @@ ShortcutEventHandler _insertHorzaontalRule = (editorState, event) {
 
 SelectionMenuItem horizontalRuleMenuItem = SelectionMenuItem(
   name: () => 'Horizontal rule',
-  icon: const Icon(Icons.horizontal_rule),
+  icon: const Icon(
+    Icons.horizontal_rule,
+    color: Colors.black,
+    size: 18.0,
+  ),
   keywords: ['horizontal rule'],
   handler: (editorState, _, __) {
     final selection =

frontend/app_flowy/packages/appflowy_editor/example/lib/plugin/tex_block_node_widget.dart

@@ -6,7 +6,11 @@ import 'package:flutter_math_fork/flutter_math.dart';
 
 SelectionMenuItem teXBlockMenuItem = SelectionMenuItem(
   name: () => 'Tex',
-  icon: const Icon(Icons.text_fields_rounded),
+  icon: const Icon(
+    Icons.text_fields_rounded,
+    color: Colors.black,
+    size: 18.0,
+  ),
   keywords: ['tex, latex, katex'],
   handler: (editorState, _, __) {
     final selection =