added services and interfaces for expense types

src/app/services/categories.ts

@@ -0,0 +1,19 @@
+import { Injectable } from '@angular/core';
+
+@Injectable({
+  providedIn: 'root',
+})
+export class Categories {
+  public static readonly BASE_URL = 'http://localhost:3000/common-cents/categories';
+
+  public async getCategories(): Promise<Category[]> {
+    console.log('getCategories called');
+
+    return [];
+  }
+}
+
+export interface Category {
+  id: string;
+  name: string;
+}