Support for prereleases released via CI.

[martnpaneq]

Suggested merge commit message (convention)

Internal: Support for releasing the repository for prereleases.

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Additional information

Part of https://github.com/cksource/ckeditor5-internal/issues/3710.

[coveralls]

Pull Request Test Coverage Report for Build 328950fa-7d07-49ed-a54f-58e8e02c16df

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build b2b09a94-0c7f-4020-b16e-6c62d273a0ec:	0.0%
Covered Lines:	166
Relevant Lines:	166

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💛 - Coveralls

[psmyrek]

There is something wrong with the headers. There is already ### Releasing (level 3) header above.

[psmyrek]

Tabs are mixed with spaces.

[pomek]

Please use the following snippet in README.

CircleCI automates the release process and can release both channels: stable (`X.Y.Z`) and pre-releases (`X.Y.Z-alpha.X`, etc.).

Before you start, you need to prepare the changelog entries.

1. Make sure the `#master` branch is up-to-date: `git fetch && git checkout master && git pull`.
1. Prepare a release branch: `git checkout -b release-[YYYYMMDD]` where `YYYYMMDD` is the current day.
1. Generate the changelog entries: `yarn run changelog --branch release-[YYYYMMDD] [--from [GIT_TAG]]`.
    * By default, the changelog generator uses the latest published tag as a starting point for collecting commits to process.

      The `--from` modifier option allows overriding the default behavior. It is required when preparing the changelog entries for the next stable release while the previous one was marked as a prerelease, e.g., `@alpha`.

      **Example**: Let's assume that the `v40.5.0-alpha.0` tag is our latest and that we want to release it on a stable channel. The `--from` modifier should be equal to `--from v40.4.0`.
    * This task checks what changed in each package and bumps the version accordingly. It won't create a new changelog entry if nothing changes at all. If changes were irrelevant (e.g., only dependencies), it would make an "_internal changes_" entry.
    * Scan the logs printed by the tool to search for errors (incorrect changelog entries). Incorrect entries (e.g., ones without the type) should be addressed. You may need to create entries for them manually. This is done directly in CHANGELOG.md (in the root directory). Make sure to verify the proposed version after you modify the changelog.
1. Commit all changes and prepare a new pull request targeting the `#master` branch.
1. Ping the `@ckeditor/ckeditor-5-devops` team to review the pull request and trigger the release process.

[psmyrek]

LGTM 🎉