Self-destruct command

There should be a command, such as ghcup nuke that will:

1. delete ~/.ghcup completely including its own binary if it resides there
2. warn about possible leftovers in bashrc/zshrc/fishconfig files
3. print a big fat warning for manual deletion if ghcup the binary is in a non-standard location

A challenge here is that, under windows, you can't delete the currently running executable. There are some tricks to work around it, though.