feat: add GitHub action outputs #401

Description

Current GitHub action has some inputs, then it does something, but it has not usable outputs to check what the results were.
As a user of the GitHub action, i want several outputs, so that i can use them programmatically in GitHub workflows.

• released: bool(-ish) value that indicates whether a released happened
• version: (new) semantic version that was calculated. if released was false than this would be the current version detected.
• tag: VCS tag-name was used for the release. if released was false than this would be empty string.
• url: Github.com URL for the release. if released is false, then this value is empty string
• id: Release ID. if released is false, then this value is null
• assets: JSON array containing information about each uploaded asset, in the format given here (minus the uploader field). if released is false, then this value is an empty JSON list
• assets-dist: JSON object containing information about each uploaded dist asset, in the format given here (minus the uploader field). if released is false, then this value is an empty JSON object
• upload_url: URL for uploading assets to the release. if released is false, then this value is an empty string

output inspired by https://github.com/softprops/action-gh-release

Use cases

• released: python-semantic-release GitHub action might detect that no release should happen. To indicate this, you can check this value. As an example, you can use ${{ steps.<step-id>.outputs.released }} to check if a release happened and control other steps based on that information. For example, a DockerHub release can be skipped, if python-semantic-release did not happen, although the GitHub action ended with success() state.
• version: As an example, you can use ${{ steps.<step-id>.outputs.version }} to get the version that was currently detected/used for the release.
• tag: As an example, you can use ${{ steps.<step-id>.outputs.tag }} as an input for https://github.com/marketplace/actions/upload-files-to-a-github-release
• assets: As an example, you can use ${{ fromJSON(steps.<step-id>.outputs.assets-dist)[0].browser_download_url }} to get the download URL of the first asset. This file can then be downloaded and put in a docker image.
• asset-dist: As an example, you can use ${{ fromJSON(steps.<step-id>.outputs.assets-dist).wheel.browser_download_url }} to get the download URL of the wheel-dist asset. This file can then be downloaded and put in a docker image.
• upload_url: As an example, you can use ${{ steps.<step-id>.outputs.upload_url }} as an input for https://github.com/actions/upload-release-asset

Possible implementation

• outputs can be defined in the https://github.com/relekang/python-semantic-release/blob/master/action.yml
• output can be issued via workflow commands on stdOut - see https://docs.github.com/en/actions/learn-github-actions/workflow-commands-for-github-actions
  ala print('::set-output name=version::1.2.3') - maybe there is a lib for that, or the sanitizers can be copied from https://github.com/actions/toolkit/blob/daf8bb00606d37ee2431d9b1596b88513dcf9c59/packages/core/src/command.ts#L80-L94
• output released should be boolean true/`` so it is usable in expressions
  Therefore, it seems like GitHub expression treats every non-empty string as true and empty string as false.
• to print the needed output for "workflow commands" only when run in GitHub action, the code could check if a certain environment variable is present in GitHub action only. Maybe have an extra log-stream just for "workflow commands" that is dis/enabled based on the env
• detection that run in a GitHub action can be done via env vars that are set by GitHub actions anyway:
  /usr/bin/docker run .... -e GITHUB_ACTIONS=true -e CI=true ....