Include the index in the property name when formatting a SequenceValue

I had previously hesitated a lot to include the index and when improving the example code in the README I stumbled on a problem with Log4View: it can't handle XML with two identical properties (no error is displayed but no log is displayed at all either).

Before this commit (unreadable with Log4View):
```
<log4net:properties>
  <log4net:data name="Args" value="--first-argument" />
  <log4net:data name="Args" value="--second-argument" />
</log4net:properties>
```

After this commit (no problem with Log4View):
```
<log4net:properties>
  <log4net:data name="Args[0]" value="--first-argument" />
  <log4net:data name="Args[1]" value="--second-argument" />
</log4net:properties>
```

Author: 0xced

src/Log4NetTextFormatter.cs

@@ -291,9 +291,9 @@ private void WriteDictionaryProperty(XmlWriter writer, string propertyName, Dict
         /// <param name="sequenceValue">The <see cref="SequenceValue"/> to write.</param>
         private void WriteSequenceProperty(XmlWriter writer, string propertyName, SequenceValue sequenceValue)
         {
-            foreach (var value in sequenceValue.Elements)
+            foreach (var (value, i) in sequenceValue.Elements.Select((e, i) => (e, i)))
             {
-                WritePropertyElement(writer, propertyName, value);
+                WritePropertyElement(writer,$"{propertyName}[{i}]", value);
             }
         }
 

tests/Log4NetTextFormatterTest.SequenceProperty.approved.xml

@@ -1,8 +1,8 @@
 <log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">
   <log4net:properties>
-    <log4net:data name="Sequence" value="1" />
-    <log4net:data name="Sequence" value="two" />
-    <log4net:data name="Sequence" value="[(1: &quot;one&quot;), (&quot;two&quot;: 2)]" />
+    <log4net:data name="Sequence[0]" value="1" />
+    <log4net:data name="Sequence[1]" value="two" />
+    <log4net:data name="Sequence[2]" value="[(1: &quot;one&quot;), (&quot;two&quot;: 2)]" />
   </log4net:properties>
   <log4net:message><![CDATA[Hello from Serilog]]></log4net:message>
 </log4net:event>