Multiline outputs support in the GitHub action

[pjxiao]

Please describe your feature request.

• Support multiline outputs in the Github Actions

• Even though yq produces a multiline output, only the first line of it is stored in the outputs.result

• Current behaviour:
  example: https://github.com/pjxiao/yq-action/runs/3577428434

  jobs:
    convert-to-json:
      runs-on: ubuntu-latest
      steps:
        - uses: actions/checkout@v2
        - name: This outputs multiline JSON
          uses: mikefarah/yq@v4.12.2
          id: act
          with:
            cmd: yq eval --output-format=json 'config.yml'
  
        - name: But we get only the first line here
          env:
            ACT_RESULT: ${{ steps.act.outputs.result }}  # = "{"
          run: echo "${ACT_RESULT}"

Describe the solution you'd like

• If yq produces a multiline output, the entire output is store in the outputs.result

• Expected behaviour:

  example: https://github.com/pjxiao/yq-action/runs/3577428423

  jobs:
    convert-to-json:
      runs-on: ubuntu-latest
      steps:
        - uses: actions/checkout@v2
        - name: This outputs multiline JSON
          uses: mikefarah/yq@v4.12.2
          id: act
          with:
            cmd: yq eval --output-format=json 'config.yml'
  
        - name: I'd like to get the entire JSON
          env:
            ACT_RESULT: ${{ steps.act.outputs.result }}
          run: echo "${ACT_RESULT}"

Additional context

• This is useful when we use yq as a YAML/JSON converter