挺大的改动。增加 as运算符，转换不了抛出 TypeError。import语法更新。修复try一点错误。现在表达式运算返回ExprResult。通过3个宏实现简便错误传播与解包 unwrap

src/Lexer/lexer.cpp

@@ -97,6 +97,7 @@ namespace Fig
         {FString(u8"catch"), TokenType::Catch},
         {FString(u8"throw"), TokenType::Throw},
         {FString(u8"Finally"), TokenType::Finally},
+        {FString(u8"as"), TokenType::As},
 
         // {FString(u8"Null"), TokenType::TypeNull},
         // {FString(u8"Int"), TokenType::TypeInt},