新增测试文件，验证现有变量声明语法，包括显式类型声明、自动类型推断及复杂表达式的类型推断，增强了类型系统的测试覆盖。

Author: HelloAIXIAOJI

test_existing_syntax.cn

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+// 测试现有的变量声明语法
+
+using ns std;
+
+fn test_variable_declarations() : void {
+    // 测试显式类型声明
+    x: int = 42;
+    y: float = 3.14;
+    z: string = "Hello";
+    
+    std::println("显式类型声明:");
+    std::println("x (int) = " + x);
+    std::println("y (float) = " + y);
+    std::println("z (string) = " + z);
+    
+    // 测试自动类型推断
+    a: auto = 100;
+    b: auto = 2.71;
+    c: auto = "World";
+    
+    std::println("自动类型推断:");
+    std::println("a (auto) = " + a);
+    std::println("b (auto) = " + b);
+    std::println("c (auto) = " + c);
+    
+    // 测试复杂表达式的类型推断
+    d: auto = x + a;
+    e: auto = y * 2.0;
+    f: auto = z + " " + c;
+    
+    std::println("复杂表达式类型推断:");
+    std::println("d = " + d);
+    std::println("e = " + e);
+    std::println("f = " + f);
+};
+
+fn main() : void {
+    test_variable_declarations();
+};