Support auto insert of raw string literals

Author: davidwengier

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/AutoInsert/AutoInsertService.cs

@@ -18,7 +18,7 @@ internal class AutoInsertService(IEnumerable<IOnAutoInsertProvider> onAutoInsert
     public static FrozenSet<string> HtmlAllowedAutoInsertTriggerCharacters { get; }
         = new string[] { "=" }.ToFrozenSet(StringComparer.Ordinal);
     public static FrozenSet<string> CSharpAllowedAutoInsertTriggerCharacters { get; }
-        = new string[] { "'", "/", "\n" }.ToFrozenSet(StringComparer.Ordinal);
+        = new string[] { "'", "/", "\n", "\"" }.ToFrozenSet(StringComparer.Ordinal);
 
     private readonly ImmutableArray<string> _triggerCharacters = CalculateTriggerCharacters(onAutoInsertProviders);
 

src/Razor/test/Microsoft.VisualStudioCode.RazorExtension.Test/Endpoints/Shared/CohostOnAutoInsertEndpointTest.cs

@@ -82,6 +82,27 @@ The end.
             delegatedResponseText: "\"$0\"");
     }
 
+    [Fact]
+    public async Task CSharp_RawStringLiteral()
+    {
+        await VerifyOnAutoInsertAsync(
+            input: """"
+                @code {
+                    void TestMethod() {
+                        var x = """$$
+                    }
+                }
+                """",
+            output: """""""
+                @code {
+                    void TestMethod() {
+                        var x = """$0"""
+                    }
+                }
+                """"""",
+            triggerCharacter: "\"");
+    }
+
     [Fact]
     public async Task CSharp_OnForwardSlash()
     {