feat: add type alias for file write operation

npm/ng-packs/packages/schematics/src/models/index.ts

@@ -5,4 +5,5 @@ export * from './model';
 export * from './project';
 export * from './proxy-config';
 export * from './service';
+export * from './tree';
 export * from './util';

npm/ng-packs/packages/schematics/src/models/tree.ts

@@ -0,0 +1 @@
+export type WriteOp = 'create' | 'overwrite';