Implement 'rad upgrade' to upgrade rad versions

[amterp]

We've recently made Rad 'go install'-able. But since we don't offer a way for rad to upgrade itself, users will be on these older versions until they themselves run 'go install' again.

https://github.com/fynelabs/selfupdate/tree/main looks interesting for this.

tbd on terminology between 'upgrade' or 'update'.

Can we detect if we're installed via go install, or via package manager? So as to reject 'upgrade' attempts if e.g. version managed by package manager.