Support user-id syntax

[srajiang]

Description

As suggested in #41, this action could support the following syntax of user-id for improved clarity. Currently channel-id accepts either a Slack CHANNEL_ID or USER_ID.

- name: Post to a Slack channel
  id: slack
  uses: slackapi/[email protected]
  with:
    user-id: 'USER_ID' # currently channel-id field accepts either CHANNEL_ID or USER_ID
    slack-message: 'posting from a github action!'
  env:
    SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}

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