v1.0.0

Add an upgrade-preview test for local commands (#483)

As the title says. Recorded via
```
provider $ go test -v github.com/pulumi/pulumi-command/provider/pkg/provider -run TestUpgradeLocalCommand/stdin/upgrade-snapshot -provider-snapshot 
```

It would be good to have one for remote commands as well but that'll
take a bit more work. All the integration tests in `examples/` that use
remote command also use `CopyToRemote` which doesn't exist in v0.*.