Avoid string allocation and improve performance of `JsonProperty.Writ…

…eTo` (dotnet#90074)

* Avoid string allocation in WriteTo when possible.

System.Text.Json.JsonProperty.WriteTo uses get_Name, calling
JsonElement.GetPropertyName() which would allocate a string.

Use ReadOnlySpan<byte> from the underlying UTF8 json, when possible,
by adding helper methods into JsonDocument & JsonElement.

Fix dotnet#88767

* Avoid alloc in unescaping & escaping.

Current code unescapes & escapes property names and uses ToArray.
Avoid alloc by adding internal GetRaw/WriteRaw methods.

* Fix bugs on escaped property names

Original code doesn't handle GetRaw/WriteRaw on escaped
property names correctly.

* Change IndexOf to Contains if possible.

* Further avoid alloc by inlining GetUnescapedSpan

Allocations are further avoided when the property name is shorter than
JsonConstants.StackallocByteThreshold, by inlining
JsonReaderHelper.GetUnescapedSpan.

* Move writing logic to JsonElement;
Shorten names of new methods;
Add a test of writing out special names.

* Move logic into JsonDocument

* fix format

* fix format 2

* improve comment

* removed unused stub

* added assertion

* removed unnecessary test

src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonDocument.cs

@@ -874,6 +874,15 @@ private static void ClearAndReturn(ArraySegment<byte> rented)
             }
         }
 
+        internal void WritePropertyName(int index, Utf8JsonWriter writer)
+        {
+            CheckNotDisposed();
+
+            DbRow row = _parsedData.Get(index - DbRow.Size);
+            Debug.Assert(row.TokenType == JsonTokenType.PropertyName);
+            WritePropertyName(row, writer);
+        }
+
         private void WritePropertyName(in DbRow row, Utf8JsonWriter writer)
         {
             ArraySegment<byte> rented = default;

src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonElement.cs

@@ -1316,6 +1316,13 @@ public void WriteTo(Utf8JsonWriter writer)
             _parent.WriteElementTo(_idx, writer);
         }
 
+        internal void WritePropertyNameTo(Utf8JsonWriter writer)
+        {
+            CheckValidInstance();
+
+            _parent.WritePropertyName(_idx, writer);
+        }
+
         /// <summary>
         ///   Get an enumerator to enumerate the values in the JSON array represented by this JsonElement.
         /// </summary>

src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonProperty.cs

@@ -117,7 +117,15 @@ public void WriteTo(Utf8JsonWriter writer)
                 ThrowHelper.ThrowArgumentNullException(nameof(writer));
             }
 
-            writer.WritePropertyName(Name);
+            if (_name is null)
+            {
+                Value.WritePropertyNameTo(writer);
+            }
+            else
+            {
+                writer.WritePropertyName(_name);
+            }
+
             Value.WriteTo(writer);
         }