Print commas at the end of the line (fluentassertions#1945)

When using line breaks, print commas behind the listed items, instead
of on a new line.

Before:
```
Expected someCollection {
    item1
    ,
    item2
    ,
```
After:
```
Expected someCollection {
    item1,
    item2,
```

Co-authored-by: ronaldkroon <[email protected]>

Author: ronaldkroon

Src/FluentAssertions/Formatting/DefaultValueFormatter.cs

@@ -42,7 +42,7 @@ public void Format(object value, FormattedObjectGraph formattedGraph, Formatting
         {
             if (context.UseLineBreaks)
             {
-                formattedGraph.AddLine(value.ToString());
+                formattedGraph.AddFragmentOnNewLine(value.ToString());
             }
             else
             {

Tests/FluentAssertions.Specs/Formatting/FormatterSpecs.cs

@@ -653,6 +653,20 @@ public void When_formatting_a_large_dictionary_it_should_limit_the_number_of_for
         result.Should().Match("*…18 more…*");
     }
 
+    [Fact]
+    public void When_formatting_multiple_items_with_a_custom_string_representation_using_line_breaks_it_should_end_lines_with_a_comma()
+    {
+        // Arrange
+        var subject = new[] { typeof(A), typeof(B) };
+
+        // Act
+        string result = Formatter.ToString(subject, new FormattingOptions { UseLineBreaks = true } );
+
+        // Assert
+        result.Should().Contain($"FluentAssertions.Specs.Formatting.FormatterSpecs+A, {Environment.NewLine}");
+        result.Should().Contain($"FluentAssertions.Specs.Formatting.FormatterSpecs+B{Environment.NewLine}");
+    }
+
     public class BaseStuff
     {
         public int StuffId { get; set; }