LibJS: Add support for arrow functions

Author: https://github.com/jack-karamanian
Commit: 098f1cd0ca
Pull-request: https://github.com/SerenityOS/serenity/pull/1532
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Lexer.cpp

@@ -86,6 +86,7 @@ Lexer::Lexer(StringView source)
     }
 
     if (s_two_char_tokens.is_empty()) {
+        s_two_char_tokens.set("=>", TokenType::Arrow);
         s_two_char_tokens.set("+=", TokenType::PlusEquals);
         s_two_char_tokens.set("-=", TokenType::MinusEquals);
         s_two_char_tokens.set("*=", TokenType::AsteriskEquals);