Install from `go install github.com/davfive/gitspaces/v2/cmd@latest`

[davfive]

Make it obvious during go install that a go executable rather than a module/package is being installed

e.g., go install github.com/davfive/gitspaces/v2/cmd@latest

Requires moving main.go into ./cmd and figuring out how to install a v2 package from a subdirectory