Take the ancestor path into account when env-completing from uncles.

[jneem]

Consider the following input, which I think follows a pretty common pattern:

{ foo = { bar = { ba‸ } } } | { foo.bar.baz | default = 1 }

(with ‸ denoting the cursor). This will use env completion, getting possible completions from the parent nodes in the AST, along with the "great-uncle" node {foo.bar.baz | default = 1}. So we should offer the bar completion because it's defined in the parent record. But we should also include baz as a completion because our great-uncle defines foo.bar.baz and our path relative to our grandparent is foo.bar.

This PR implements some extra path tracking when traversing the parent chain, adding these extra completions.

As an extra small thing, it discards completions if their only occurence is at the cursor itself. This prevents us from offering ba as a completion in the example above.