feat: set env vars for outputs

[jenseng]

Fixes #17

Optionally set env vars for each of the action's outputs. This provides a simpler method to access outputs than the default fromJSON approach.

For example, instead of:

- id: setup_node
  uses: jenseng/dynamic-uses@v1
  with:
    uses: actions/setup-node@${{ inputs.version }}
    with: |
      node-version: 18
- env:
    # pull the node-version out of the outputs
    node_version: ${{ fromJSON(steps.setup_node.outputs.outputs).node-version }}
  run: echo "Installed $node_version"

You can set env-outputs: true like so:

- uses: jenseng/dynamic-uses@v1
  with:
    uses: actions/setup-node@${{ inputs.version }}
    with: |
      node-version: 18
    env-outputs: true # this will set a node_version env var for the node-version output
- run: echo "Installed $node_version"
  # it can also be used in subsequent step expressions, e.g. `env.node_version`

For more control over the output variable names and how to deal with potential conflicts, see the new env-outputs-prefix, env-outputs-upcase, and env-outputs-on-conflict options documented in the README, metadata file, and tests.