content [nfc]: Use toDiagnosticsNode(name: ...)

This produces slightly nicer output, by indicating the index for
each item.  For example, after introducing an error in one test,
the expected and actual values it prints look like this:

    Actual: <ZulipContent
     └─ListNode
       │ unordered
       │
       ├─item 0: BlockContentNode list
       │ └─ParagraphNode
       │   │ was implicit
       │   │
       │   └─TextNode
       │       "something"
       │
       └─item 1: BlockContentNode list
         └─ParagraphNode
           │ was implicit
           │
           └─TextNode
               "another">

instead of like this:

    Actual: <ZulipContent
     └─ListNode
       │ unordered
       │
       ├─list item
       │ └─ParagraphNode
       │   │ was implicit
       │   │
       │   └─TextNode
       │       "something"
       │
       └─list item
         └─ParagraphNode
           │ was implicit
           │
           └─TextNode
               "another">

This also means less code as we start adding more nodes that have
this kind of structure; it avoids having to make a new subclass of
DiagnosticableTree for each one.

Author: gnprice

lib/model/content.dart

@@ -1,3 +1,4 @@
+import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 import 'package:html/dom.dart' as dom;
 import 'package:html/parser.dart';
@@ -113,6 +114,20 @@ class UnimplementedBlockContentNode extends BlockContentNode
   // No ==/hashCode, because htmlNode is a whole subtree.
 }
 
+class _BlockContentListNode extends DiagnosticableTree {
+  const _BlockContentListNode(this.nodes);
+
+  final List<BlockContentNode> nodes;
+
+  @override
+  String toStringShort() => 'BlockContentNode list';
+
+  @override
+  List<DiagnosticsNode> debugDescribeChildren() {
+    return nodes.map((node) => node.toDiagnosticsNode()).toList();
+  }
+}
+
 /// A block content node whose children are inline content nodes.
 ///
 /// A node of this type expects a block layout context from its parent,
@@ -226,25 +241,12 @@ class ListNode extends BlockContentNode {
   @override
   List<DiagnosticsNode> debugDescribeChildren() {
     return items
-      .map((nodes) => _ListItemDiagnosticableNode(nodes).toDiagnosticsNode())
+      .mapIndexed((i, nodes) =>
+        _BlockContentListNode(nodes).toDiagnosticsNode(name: 'item $i'))
       .toList();
   }
 }
 
-class _ListItemDiagnosticableNode extends DiagnosticableTree {
-  _ListItemDiagnosticableNode(this.nodes);
-
-  final List<BlockContentNode> nodes;
-
-  @override
-  String toStringShort() => 'list item';
-
-  @override
-  List<DiagnosticsNode> debugDescribeChildren() {
-    return nodes.map((node) => node.toDiagnosticsNode()).toList();
-  }
-}
-
 class QuotationNode extends BlockContentNode {
   const QuotationNode(this.nodes, {super.debugHtmlNode});