LibCpp: Support Lexing escaped newlines

Author: https://github.com/itamar8910
Commit: https://github.com/SerenityOS/serenity/commit/1bd6705636b
Pull-request: https://github.com/SerenityOS/serenity/pull/6916
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibCpp/Lexer.cpp

@@ -757,6 +757,13 @@ Vector<Token> Lexer::lex()
                 commit_token(Token::Type::Identifier);
             continue;
         }
+
+        if (ch == '\\' && peek(1) == '\n') {
+            consume();
+            consume();
+            continue;
+        }
+
         dbgln("Unimplemented token character: {}", ch);
         emit_single_char_token(Token::Type::Unknown);
     }