We used to do the equivalent of `rootCmd.SetOut(os.Stdout)` because we
thought that Cobra's "Out" stream represents standard output. However,
upon closer inspection it turns out that this is Cobra's stream for
usage errors and deprecation warnings, and those we want written to
stderr as well. It is not clear to me why Cobra maintains a distinction
between "Out" and "Err" streams since both seem to go to sdterr by
default.
This change also ceases our usage of `command.Print()` functions in
favor of explicitly writing to `IOStreams.Out/ErrOut`.
Replace the implementation that relied on symlinks with the one that
create regular files that act like symlinks: they contain a reference to
the local directory where to find the extension.
- Extensions on Windows now enabled through the `sh.exe` interpreter
- `sh.exe` now found on Windows when git was installed via scoop
- `gh extensions list` command shows origin repo for the extension
- `gh extensions upgrade --all` is required to upgrade all extensions
- Added `gh extensions remove`
- Shell completions now include aliases and extension names
- `gh` help output now lists available extension names
- Extensions are stored to XDG_DATA_HOME
