Remove a number of unwraps. In some cases, a `?` just worked as is. I also made it possible to use `?` to go from Result<OutputStream, ShellError> to OutputStream. Finally, started updating PerItemCommand to be able to use the signature deserialization logic, which substantially reduces unwraps.
This is still in-progress work, but tests pass and it should be clear to merge and keep iterating on master.
This commit makes it possible to force nu to treat a command as an external command by prefixing it with `^`. For example `^dir` will force `dir` to run an external command, even if `dir` is also a registered nu command.
This ensures that users don't need to leave nu just because we happened to use a command they need.
This commit adds a new token type for external commands, which, among other things, makes it pretty straight forward to syntax highlight external commands uniquely, and generally to treat them as special.
