Add AllowsKeywordRecommender to KeywordCompletionProvider (#73608)

Without this completion for `allows` keyword doesn't work in IDE

Author: AlekseyTs

src/Features/CSharp/Portable/Completion/CompletionProviders/KeywordCompletionProvider.cs

@@ -28,6 +28,7 @@ public KeywordCompletionProvider()
             new AbstractKeywordRecommender(),
             new AddKeywordRecommender(),
             new AliasKeywordRecommender(),
+            new AllowsKeywordRecommender(),
             new AndKeywordRecommender(),
             new AnnotationsKeywordRecommender(),
             new AscendingKeywordRecommender(),