use tsconfig.json#include for vscode.d.ts and vscode.proposed.d.ts, enables to remove most ref.d.ts-files

extensions/vscode-custom-editor-tests/src/test/customEditor.test.ts

@@ -3,6 +3,7 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 
+import 'mocha';
 import * as assert from 'assert';
 import * as fs from 'fs';
 import * as path from 'path';