A semi-empty var is one with a single empty string element. The
`env_var_t::empty()` method returns true for such vars but we want
`set --show` to report that it has a single empty element.
This adds a new capability to the `set` command. It is similar to
running `set` with no other arguments but provides far more detail about
each variable. Such as whether it is set in each of the local, global,
and universal scopes. And the values in each scope. You can also ask for
specific variables to be shown.
Fixes#4265
