Parser: emit warnings on non-existent YAML configuration keys

[edigaryev]

To avoid things like cirruslabs/cirrus-ci-docs#1043.

[edigaryev]

Took a look at this, and it seems that some refactoring will be needed first to pack this change in a concise PR.

Namely, we have two places where a virtual *node.Node is emitted:

• collectible fields
• YAML anchors

We need to ensure a 1-to-1 mapping between *node.Node and yaml.Node somehow, otherwise used/unused node bookkeeping becomes very hard go grasp.