To match the convention of `helm install`, `helm uninstall` is the inverse.

Other tangential changes in this PR:

- StatusDeleting has been changed to StatusUninstalling
- StatusDeleted has been changed to StatusUninstalled
- `helm list --deleted` has been changed to `helm list --uninstalled`
- `helm list --deleting` has been changed to `helm list --uninstalling`
- `helm.DeleteOption` and all delete options have been renamed to `helm.UninstallOption`

I have not made any changes to the "helm.sh/hook-delete-policy", "pre-delete" and "post-delete" hook annotations because

1. it's a major breaking change to existing helm charts, which we've commited to NOT break in Helm 3
2. there is no "helm.sh/hook-install-policy" to pair with "helm.sh/hook-uninstall-policy", so delete still makes sense here

`helm delete` and `helm del` have been added as aliases to `helm uninstall`, so `helm delete` and `helm del` still works as is.