table designer editor icon (#17608)

* icon for table designer

* use more specific name

* use sql defined schema

src/sql/base/common/schemas.ts

@@ -0,0 +1,11 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+export namespace Schemas {
+	/**
+	 * A schema that is used for table designer editor
+	 */
+	export const tableDesigner = 'ads-table-designer';
+}