Allow `if` Blocks in steps

[fchorney]

Describe the enhancement
I'm finding that it is quite annoying to have to define an if condition for every single item in the steps section of a test matrix. It would be great if you could define a block per se

Code Snippet
For instance, let's say I have a setup like so

      # Linux
      - name: Wait / Sleep
        uses: jakejarvis/wait-action@v0.1.0
        if: ${{ runner.os == 'Linux' }}
        with:
          time: '1m'
      - name: Collect Docker Logs
        uses: jwalton/gh-docker-logs@v1.0.0
        if: ${{ runner.os == 'Linux' }}
      - name: Check running containers
        run: docker ps -a
        if: ${{ runner.os == 'Linux' }}
      - name: Set PGHOST on Linux
        run: echo "PGHOST=localhost" >> $GITHUB_ENV
        if: ${{ runner.os == 'Linux' }}
      # MacOS
      - name: Set PGUSER on macOS
        run: |
          echo "PGUSER=$USER" >> $GITHUB_ENV
          echo "LIBPQJL_DATABASE_USER=$USER" >> $GITHUB_ENV
        if: ${{ runner.os == 'macOS' }}
      - name: Start Homebrew PostgreSQL service
        run: pg_ctl -D /usr/local/var/postgres start
        if: ${{ runner.os == 'macOS' }}
      # Windows
      - name: Add PostgreSQL to Path
        run: |
          echo $env:PGBIN
          echo $env:PGBIN | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
        if: ${{ runner.os == 'Windows' }}
      - name: Start Windows PostgreSQL service
        run: |
          pg_ctl -D $env:PGDATA start
          pg_ctl -D $env:PGDATA status
        if: ${{ runner.os == 'Windows' }}

It would be awesome if it could be formatted like so:

      # Linux
      - if: ${{ runner.os == 'Linux' }}
        - name: Wait / Sleep
          uses: jakejarvis/wait-action@v0.1.0
          with:
            time: '1m'
        - name: Collect Docker Logs
          uses: jwalton/gh-docker-logs@v1.0.0
        - name: Check running containers
          run: docker ps -a
        - name: Set PGHOST on Linux
          run: echo "PGHOST=localhost" >> $GITHUB_ENV
      # MacOS
      - if: ${{ runner.os == 'macOS' }}
        - name: Set PGUSER on macOS
          run: |
            echo "PGUSER=$USER" >> $GITHUB_ENV
            echo "LIBPQJL_DATABASE_USER=$USER" >> $GITHUB_ENV
        - name: Start Homebrew PostgreSQL service
          run: pg_ctl -D /usr/local/var/postgres start
      # Windows
      - if: ${{ runner.os == 'Windows' }}
        - name: Add PostgreSQL to Path
          run: |
            echo $env:PGBIN
            echo $env:PGBIN | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
        - name: Start Windows PostgreSQL service
          run: |
            pg_ctl -D $env:PGDATA start
            pg_ctl -D $env:PGDATA status