feat: support extending a parent environment (#398)
Adds a new environment configuration to bin/hermit.hcl
```
inherit-parent = true
```
Setting this inherits the Hermit environment from a parent directory,
inheriting environment variables and packages.
This should be useful in larger monorepos where we want to have a single
root environment with most tooling, but want to still allow subprojects
to have their own owerwrites and environment variables.
Note, the packages in parent environment won't have access to the child
environment even when being called from the child environment. This
means that any package calling another package in the parent environment
will still call the parent package even if that is overwritten in the
child environment. Though this can be potentially confusing, it should
give more consistent behaviour between parent and child environments.
Also, package dependencies can not cross environments. So, it is not
possible to have a package dependency from a child environment package
to a parent environment package.
Finally, for the time being, all hermit operations apply only to the
currently active environment. So, it is not possible to upgrade,
install, or uninstall parent packages from the child environment
