fix(lexer): add false, true and null keywords

src/keyword.py

@@ -10,5 +10,8 @@ KEYWORDS: dict[str, TokenType] = {
     "while": TokenType.WHILE,
     "from": TokenType.FROM,
     "to": TokenType.TO,
-    "by": TokenType.BY
+    "by": TokenType.BY,
+    "false": TokenType.FALSE,
+    "true": TokenType.TRUE,
+    "null": TokenType.NULL,
 }

src/token.py

@@ -40,6 +40,7 @@ class TokenType(Enum):
     NUMBER = auto()
     TRUE = auto()
     FALSE = auto()
+    NULL = auto()
 
     # Keywords
     LET = auto()