feat(showcase): add aurelia 2 calculator showcase

- Introduced a new showcase entry for the Aurelia 2 Calculator, including an overview and relevant metadata. Added a screenshot to enhance visual appeal.

Author: Vheissu

content/showcase/au2-calculator/index.md

@@ -0,0 +1,11 @@
+---
+title: "Aurelia 2 Calculator"
+description: "A simple calculator built with Aurelia 2, featuring an elegant UI and basic operations."
+date: 2024-07-11
+showcase_categories: ["apps"]
+github: "Vheissu/aurelia2-calculator"
+---
+
+## Overview
+
+Aurelia 2 Calculator is a simple calculator built with Aurelia 2. It features an elegant UI and basic operations. Perfect for learning Aurelia 2 basics while creating something useful and interactive.