fix(parser): minor improvements for incomplete statements

src/parser/parser.py

@@ -18,7 +18,7 @@ class Parser:
     }
 
     STATEMENT_BOUNDARY: set[TokenType] = {
-        TokenType.FOR, TokenType.WHILE, TokenType.IF
+        TokenType.FOR, TokenType.WHILE, TokenType.IF, TokenType.CLASS, TokenType.FUN, TokenType.LET, TokenType.RETURN
     }
 
     def __init__(self, tokens: list[Token]):
@@ -51,10 +51,10 @@ class Parser:
             return False
         return self.peek().type == token_type
 
-    def advance(self):
-        token: Token = self.peek()
-        self.current += 1
-        return token
+    def advance(self) -> Token:
+        if not self.is_at_end():
+            self.current += 1
+        return self.previous()
 
     def match(self, *types: TokenType) -> bool:
         for token_type in types: