[web] Split the EngineParagraph interface from the legacy implementation (#22307)

Author: mdebbar

ci/licenses_golden/licenses_flutter

@@ -526,6 +526,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/services/message_codecs.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/services/serialization.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/shadow.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/test_embedding.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/text/canvas_paragraph.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/text/font_collection.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/text/line_break_properties.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/text/line_breaker.dart

lib/web_ui/lib/src/engine.dart

@@ -129,6 +129,7 @@ part 'engine/text/line_break_properties.dart';
 part 'engine/text/line_breaker.dart';
 part 'engine/text/measurement.dart';
 part 'engine/text/paragraph.dart';
+part 'engine/text/canvas_paragraph.dart';
 part 'engine/text/ruler.dart';
 part 'engine/text/unicode_range.dart';
 part 'engine/text/word_break_properties.dart';

lib/web_ui/lib/src/engine/html/recording_canvas.dart

@@ -1185,7 +1185,7 @@ class PaintDrawParagraph extends DrawCommand {
   @override
   String toString() {
     if (assertionsEnabled) {
-      return 'DrawParagraph(${paragraph._plainText}, $offset)';
+      return 'DrawParagraph(${paragraph.toPlainText()}, $offset)';
     } else {
       return super.toString();
     }