Add 'add extension completion' support for modern extensions. (#81239)

Author: CyrusNajmabadi

src/EditorFeatures/CSharpTest/Completion/CompletionProviders/CompletionProviderOrderTests.cs

@@ -62,7 +62,7 @@ public void TestCompletionProviderOrder()
             typeof(InternalsVisibleToCompletionProvider),
             typeof(PropertySubpatternCompletionProvider),
             typeof(TypeImportCompletionProvider),
-            typeof(ExtensionMethodImportCompletionProvider),
+            typeof(ExtensionMemberImportCompletionProvider),
             typeof(AggregateEmbeddedLanguageCompletionProvider),
             typeof(FunctionPointerUnmanagedCallingConventionCompletionProvider),
             typeof(CSharpSnippetCompletionProvider),