Use --no-echo, not --slave

`--slave` is deprecated since R 4.0.0:

https://github.com/r-devel/r-svn/blob/9c6e0b9b9b8475704ead72c5077c8c875d51599e/src/main/CommandLineArgs.c#L156

It technically still works, but that might change at any time. Since 4.0.0 is already 5 years old, can we just move to `--no-echo`?

https://github.com/search?q=repo%3AREditorSupport%2Fvscode-R%20%2Fslave%2F&type=code