Enable JSON validation in string literal

I've also taken this opportunity to the new raw string literal syntax
[1] so that all the double quotes in the string no longer need to be
escaped.

[1] dotnet/csharplang#4304

src/Buttercup.Web.Tests/Infrastructure/AssetManifestReaderTests.cs

@@ -8,7 +8,8 @@ public sealed class AssetManifestReaderTests
     [Fact]
     public void ReturnsManifestDeserializedFromJson()
     {
-        var bytes = Encoding.UTF8.GetBytes("{\"alpha\":\"beta\",\"gamma\":\"delta\"}");
+        var bytes = Encoding.UTF8.GetBytes(
+            /*lang=json,strict*/ """{"alpha": "beta", "gamma":"delta"}""");
 
         using var stream = new MemoryStream(bytes);