Added functionality for Welcome Message
Very cute uWu

I set the default message to Welcome to SimpleSerialShell.
To change it, one has to use the function shell.setWelcomeMessage(F("NotSimpleSerialShell"); before calling shell.attach(Serial);.
I'm not sure this change is worthwhile. Supporting a customized startup message makes the code less SIMPLE. Also, the proposed change breaks regression tests (which do not expect a welcome message).
There is an easy workaround: after shell.attach(...) just add a println(...) to print the welcome message:
...
shell.attach(Serial);
shell.println(F("Welcome to my Shell"));
...
I do something similar with my showID() commands in the examples.
That said, I think you do understand how the shell works, how to make a code fork, and how to make changes to suit your needs. Your fork lets you adjust the library to your needs.
I'm going to close this out, as it's not needed.