Don't include `public` keyword in completion inside an interface

Fix #732. C# completion list must not suggest `public` as a potential
accessibility modifier to use inside an `interface { }` block;
otherwise, it produces code that does not compile.

src/EditorFeatures/CSharpTest/Recommendations/PublicKeywordRecommenderTests.cs

@@ -183,7 +183,7 @@ public void InsideStruct()
         [Fact, Trait(Traits.Feature, Traits.Features.KeywordRecommending)]
         public void InsideInterface()
         {
-            VerifyKeyword(
+            VerifyAbsence(
 @"interface I {
    $$");
         }

src/Features/CSharp/Completion/KeywordRecommenders/PublicKeywordRecommender.cs

@@ -26,7 +26,7 @@ private static bool IsValidContextForMember(CSharpSyntaxContext context, Cancell
             if (context.SyntaxTree.IsGlobalMemberDeclarationContext(context.Position, SyntaxKindSet.AllGlobalMemberModifiers, cancellationToken) ||
                 context.IsMemberDeclarationContext(
                     validModifiers: SyntaxKindSet.AllMemberModifiers,
-                    validTypeDeclarations: SyntaxKindSet.ClassInterfaceStructTypeDeclarations,
+                    validTypeDeclarations: SyntaxKindSet.ClassStructTypeDeclarations,
                     canBePartial: false,
                     cancellationToken: cancellationToken))
             {