Add support for output status

[mheap]

#23 requested the ability to change the exit code to neutral to mark the check as skipped. Unfortunately GH Actions hasn't supported this for a while (see #31), and it's hard to check the exit code in Actions.

This PR adds a new output which can be used to check the actual status of the action when exit_type is set to success.

You can set exit_type to success then inspect outputs.status to see if the action passed or failed. This is useful when you want to perform additional actions if a label is not present, but not fail the entire build.

If the action passed, outputs.status will be success. If it failed, outputs.status will be failure.

Here is a complete workflow example for this use case:

name: Pull Request Labels
on:
  pull_request:
    types: [opened, labeled, unlabeled, synchronize]
jobs:
  label:
    runs-on: ubuntu-latest
    outputs:
      status: ${{ steps.check-labels.outputs.status }}
    steps:
      - id: check-labels
        uses: mheap/github-action-required-labels@v2
        with:
          mode: exactly
          count: 1
          labels: "semver:patch, semver:minor, semver:major"
          exit_type: success
  do-other:
    runs-on: ubuntu-latest
    needs: label
    steps:
      - run: echo SUCCESS
        if: needs.label.outputs.status == 'success'
      - run: echo FAILURE && exit 1
        if: needs.label.outputs.status == 'failure'