LibJS: Remove UndefinedLiteral, add undefined to global object

There is no such thing as a "undefined literal" in JS - undefined is
just a property on the global object with a value of undefined.
This is pretty similar to NaN.

var undefined = "foo"; is a perfectly fine AssignmentExpression :^)

Libraries/LibJS/Lexer.cpp

@@ -69,7 +69,6 @@ Lexer::Lexer(StringView source)
         s_keywords.set("true", TokenType::BoolLiteral);
         s_keywords.set("try", TokenType::Try);
         s_keywords.set("typeof", TokenType::Typeof);
-        s_keywords.set("undefined", TokenType::UndefinedLiteral);
         s_keywords.set("var", TokenType::Var);
         s_keywords.set("void", TokenType::Void);
         s_keywords.set("while", TokenType::While);