[Investigate] Pre/Post Support

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Currently pre and post steps are ignored by @github/local-action. It would be useful to implement these. Initially, they would only support running other JavaScript action code.

runs:
  using: node20
  main: main.js
  post: cleanup.js

It is important to note that information can be passed between pre, post, and "main" using GITHUB_STATE/getState/saveState (ref).