[release/dev18.0] Fix XAML peek in VS editor. XamlPeekableItemFactory needs to implement exported interface. (#80923)

Backport of #80919 to release/dev18.0

/cc @jasonmalinowski @spadapet

## Customer Impact

## Regression

- [ ] Yes
- [ ] No

[If yes, specify when the regression was introduced. Provide the PR or
commit if known.]

## Testing

[How was the fix verified? How was the issue missed previously? What
tests were added?]

## Risk

[High/Medium/Low. Justify the indication by mentioning how risks were
measured and addressed.]

Author: phil-allen-msft

src/VisualStudio/Xaml/Impl/Features/Peek/IXamlPeekableItemFactory.cs

@@ -30,4 +30,6 @@ internal interface IXamlPeekableItemFactory
 internal sealed class XamlPeekableItemFactory(
     IMetadataAsSourceFileService metadataAsSourceFileService,
     IGlobalOptionService globalOptions,
-    IThreadingContext threadingContext) : PeekableItemFactory(metadataAsSourceFileService, globalOptions, threadingContext);
+    IThreadingContext threadingContext)
+    : PeekableItemFactory(metadataAsSourceFileService, globalOptions, threadingContext)
+    , IXamlPeekableItemFactory;