Add default (C) in editoryconfig (#5998)

.editorconfig

@@ -36,6 +36,9 @@ charset = utf-8-bom
 # .NET Coding Conventions     #
 ###############################
 [*.{cs,vb}]
+# Copyright notice
+file_header_template = Copyright (c) Microsoft Corporation. All rights reserved.\nLicensed under the MIT License.
+
 # Organize usings
 dotnet_sort_system_directives_first = true
 dotnet_separate_import_directive_groups = false