anchors: Add new rule to detect undeclared or duplicated anchors

[adrienverge]

According to the YAML specification ^1:

• It is an error for an alias node to use an anchor that does not previously occur in the document.

  The forbid-undeclared-aliases option checks that aliases do have a matching anchor declared previously in the document. Since this is required by the YAML spec, this option is enabled by default.

• The alias refers to the most recent preceding node having the same anchor.

  This means that having a same anchor repeated in a document is allowed. However users could want to avoid this, so the new option forbid-duplicated-anchors allows that. It's disabled by default.

• It is not an error to specify an anchor that is not used by any alias node.

  This means that it's OK to declare anchors but don't have any alias referencing them. However users could want to avoid this, so a new option (e.g. forbid-unused-anchors) could be implemented in the future. See https://github.com/adrienverge/yamllint/pull/537.

Fixes #395 Closes #420

[coveralls]

Coverage: 99.196% (+0.01%) from 99.184% when pulling 9e67d02184b87402aae6cccf2c69f3f11fd6df44 on feat/rule-anchors into 15f820442788d6015793eddb8baced7f061fd19b on master.

[adrienverge]

I'll merge this now so it can go in the same release as https://github.com/adrienverge/yamllint/pull/548#issuecomment-1476606732.

[amimas]

Thanks for merging it. Apologies for the delay here. Have been busy lately and didn't get a chance to look into it further.