First-time contributor github action

[talldan]

Description

A github action that automatically labels opened pull requests with the 'First-time Contributor' label.

How has this been tested?

Testing isn't straightforward, requires docker locally but here are some steps!

1. Create a fork of the gutenberg repo.
2. Create a pull request on the fork using any branch.
3. Clone the fork locally.
4. In the folder for the clone, create a file event.json with the following content:

{
  "number":"1",
  "pull_request": {
    "user": {
      "login":"asdfasdfasd"
    }
  }
}

Make sure the number field represents the number for the PR created in step 2. Make the login field the random name of a user who has never committed to your fork of gutenberg.

5. Generate a personal access token with the public_repo permission.
6. Run the following commands, replacing INSERT_TOKEN_HERE with your token from step 5 and INSERT_REPO_HERE with the name of your fork from step 1 (e.g. talldan/gutenberg)

docker build .github/actions/first-time-contributor/ -t first-time-contributor
docker run -v `pwd`:`pwd` -w `pwd` -e GITHUB_EVENT_PATH=event.json -e GITHUB_TOKEN=INSERT_TOKEN_HERE -e GITHUB_REPOSITORY=INSERT_REPO_HERE -it first-time-contributor

Screenshots

Types of changes

New feature (non-breaking change which adds functionality)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[talldan]

I'm guessing it's ok for two workflows to use the same action?

[aduth]

Mixed whitespace here.