feat: 类型别名 type alias (P1 #10)

- lexer: TOK_TYPE 关键字
- ast: AST_TYPE_ALIAS + AST_PROGRAM aliases数组
- parser: parse_type_expr() 抽取, type Name = Type; 解析
- sema: 别名注册+解析, 类型标注/struct字段/函数参数均支持
- 新增测试: 15_type_alias.l, 16_type_alias_struct.l

测试: 112 通过 (41+15+41+15)

test/programs/15_type_alias.l

@@ -0,0 +1,6 @@
+type Meters = i64;
+fn main() -> i64 {
+    let x: Meters = 100;
+    print_i64(x);
+    return 0;
+}

test/programs/16_type_alias_struct.l

@@ -0,0 +1,8 @@
+struct Point { x: i64, y: i64 }
+type P = Point;
+fn main() -> i64 {
+    let p: P = Point { x: 10, y: 20 };
+    print_i64(p.x);
+    print_i64(p.y);
+    return 0;
+}