Add source location tracking to AST nodes

The commit expands the source location tracking through the AST by:

1. Switching from raw Expr/Stmt nodes to AstNode wrappers containing
source slices 2. Updating interpreter and parser to preserve location
info through node traversal 3. Enhancing error reporting with richer
source context information

src/frontend/lexer.rs

@@ -22,6 +22,7 @@ fn get_keyword_token(word: &str) -> Option<TokenType> {
         "for" => Some(TokenType::For),
         "fun" => Some(TokenType::Fun),
         "if" => Some(TokenType::If),
+        "then" => Some(TokenType::Then),
         "elif" => Some(TokenType::Elif),
         "else" => Some(TokenType::Else),
         "or" => Some(TokenType::Or),