TF-3424 Fix TMail web should display text/plain with a fixed width font

core/lib/presentation/utils/html_transformer/text/persist_preformatted_text_transformer.dart

@@ -0,0 +1,12 @@
+import 'dart:convert';
+
+import 'package:core/presentation/utils/html_transformer/base/text_transformer.dart';
+
+class PersistPreformattedTextTransformer extends TextTransformer {
+  const PersistPreformattedTextTransformer();
+
+  @override
+  String process(String text, HtmlEscape htmlEscape) {
+    return '<pre>$text</pre>';
+  }
+}