* Add Path specialization that can be extended with cty.Path;
* Point label validation to offending value;
* Fix panic if label is not a string.

```yaml
...
vars:
  labels:
    good: "clint"
    bad: [1, 2, 3]
    ugly: true
```

```sh
Error: vars.labels must be a map of strings
24:   labels:
```

```
Error: vars.labels must be a map of strings
26:     bad: [1, 2, 3]
        ^
Error: vars.labels must be a map of strings
27:     ugly: true
        ^
```