[bitbucket] parallel steps without the explicit 'steps' property fail to convert

This is a valid bitbucket-pipelines.yaml file:

image: atlassian/default-image:3

pipelines:
  default:
    - parallel:
      - step:
          name: 'Build and Test'
          script:
            - echo "Your build and test goes here..."
      - step:
          name: 'Lint'
          script:
            - echo "Your linting goes here..."
      - step:
          name: 'Security scan'
          script:
            - echo "Your security scan goes here..."

Conversion fails with this error:

./go-convert bitbucket bitbucket-pipelines.yml
2023/03/06 14:46:36 yaml: unmarshal errors:
  line 6: cannot unmarshal !!seq into yaml.Parallel

The current conversion logic requires a steps property beneath the parallel property.

This yaml converts successfully:

image: atlassian/default-image:3

pipelines:
  default:
    - parallel:
        steps:
        - step:
            name: 'Build and Test'
            script:
              - echo "Your build and test goes here..."
        - step:
            name: 'Lint'
            script:
              - echo "Your linting goes here..."
        - step:
            name: 'Security scan'
            script:
              - echo "Your security scan goes here..."

./go-convert bitbucket bitbucket-pipelines.yml 
stages:
- name: build
  spec:
    steps:
    - name: parallel
      spec:
        steps:
        - name: Build and Test
          spec:
            image: atlassian/default-image:3
            run: echo "Your build and test goes here..."
          type: script
        - name: Lint
          spec:
            image: atlassian/default-image:3
            run: echo "Your linting goes here..."
          type: script
        - name: Security scan
          spec:
            image: atlassian/default-image:3
            run: echo "Your security scan goes here..."
          type: script
      type: parallel
  type: ci
version: 1

We should update the logic to allow steps to be optional.

Start at the go-convert bitbucket bitbucket-pipelines.yml entry point and locate the Bitbucket parallel-step parsing and conversion logic. Use the first YAML example as a regression case; done means both parallel forms, with and without an explicit steps property, convert successfully to the expected Drone format.