Refactor environment to use stack-based scoping

The commit rewrites the environment to use a stack of hash maps for
managing variable scopes. This replaces the old parent-reference
approach with a simpler and more efficient stack-based model.

Key changes: - Rename Environment to EnvironmentStack - Store scopes in
a Vec of HashMaps - Add push/pop scope operations for block handling -
Update interpreter to properly manage scope lifetimes - Clean up error
handling with helper functions

src/frontend/lexer.rs

@@ -26,6 +26,7 @@ fn get_keyword_token(word: &str) -> Option<TokenType> {
         "elif" => Some(TokenType::Elif),
         "else" => Some(TokenType::Else),
         "or" => Some(TokenType::Or),
+        "in" => Some(TokenType::In),
         "print" => Some(TokenType::Print),
         "return" => Some(TokenType::Return),
         "super" => Some(TokenType::Super),
@@ -146,6 +147,7 @@ impl Lexer {
             ('+', _) => Ok(Some(self.make_token(TokenType::Plus))),
             (';', _) => Ok(Some(self.make_token(TokenType::Semicolon))),
             ('*', _) => Ok(Some(self.make_token(TokenType::Star))),
+            ('%', _) => Ok(Some(self.make_token(TokenType::Percent))),
             ('!', '=') => {
                 self.advance(); // consuma il '='
                 Ok(Some(self.make_token(TokenType::BangEqual)))