[Feature request] Add support to lint custom action files

[tddschn]

Custom action files?

Actions authors put their custom action files (action.yml in the root directory by convention, but could be anywhere with any names) in GitHub repositories so that other can use those custom actions with the uses syntax.

However, actionlint currently doesn't seem to understand the custom action yaml files, and complain about things like missing jobs section.

Why?

actionlint enables workflow authors to write workflow files confidently, and catches errors early.
It would also greatly benefit action authors if actionlint supports linting custom action files.

[rhysd]

Duplicate of #46. It would be nice to have, but there is not enough velocity to implement.