Support emphasis (#9137)

* support strikethrough

Author: lyndaidaii

src/NuGetGallery/Services/MarkdownService.cs

@@ -11,6 +11,7 @@
 using CommonMark;
 using CommonMark.Syntax;
 using Markdig;
+using Markdig.Extensions.EmphasisExtras;
 using Markdig.Parsers;
 using Markdig.Renderers;
 using Markdig.Syntax;
@@ -203,6 +204,7 @@ private RenderedMarkdownResult GetHtmlFromMarkdownMarkdig(string markdownString,
                 .UseEmojiAndSmiley()
                 .UseAutoLinks()
                 .UseReferralLinks("noopener noreferrer nofollow")
+                .UseEmphasisExtras(EmphasisExtraOptions.Strikethrough)
                 .DisableHtml() //block inline html
                 .UseBootstrap()
                 .Build();

tests/NuGetGallery.Facts/Services/MarkdownServiceFacts.cs

@@ -234,6 +234,16 @@ public void TestToHtmlWithAutoLinks()
                 Assert.Equal(expectedHtml, readMeResult.Content);
                 Assert.False(readMeResult.ImagesRewritten);
             }
+
+            [Theory]
+            [InlineData("Hello ~~world~~", "<p>Hello <del>world</del></p>")]
+            public void TestToHtmlWithStrikethrough(string originalMd, string expectedHtml)
+            {
+                _featureFlagService.Setup(x => x.IsMarkdigMdRenderingEnabled()).Returns(true);
+                var readMeResult = _markdownService.GetHtmlFromMarkdown(originalMd);
+                Assert.Equal(expectedHtml, readMeResult.Content);
+                Assert.False(readMeResult.ImagesRewritten);
+            }
         }
     }
 }