Support yaml anchors

[Jrc356]

YAML anchors have finally dropped
actions/runner#1182 (comment)

I tried using them for some path triggers such as:

on:
  push:
    paths: &paths
      - "./path/to/file"
      - "./path/to/another/file"
  pull_request:
    paths: *paths

but actionlint yells about usage of the anchors:

"paths" section must be sequence node but got alias node with "" tag

[muzimuzhi]

Duplicate of #133.

There is already an open PR to address this, see #557.