Standardize version bumping and the release process

[martnpaneq]

🚀 Summary

Bumping the version here meant editing four files by hand, and there was no changelog or tags. This adds a standard release process.

While working on a change: run pnpm nice and describe the change in the generated file. It gets committed with the PR.

📌 Related issues

Closes https://github.com/ckeditor/ckeditor5-internal/issues/4642.

📬 Releasing (three steps, from main):

1. pnpm release:prepare-changelog — turns the collected entries into a CHANGELOG.md section and commits it. You pick the new version, the suggestion is based on the entry types.
2. pnpm release:prepare-packages — writes the version to all four files and creates the release commit with a vX.Y.Z tag. Still local, so there's time to look it over.
3. pnpm release:publish-packages — pushes main plus the tag and opens the GitHub release page.

Version bumping is scripted, so package.json, plugin.json, marketplace.json and every SKILL.md can no longer drift apart — the release also refuses to start if they already have. Nothing goes to npm; the skills are still consumed straight from the repository.

The whole thing is documented in docs/release-process.md. CHANGELOG.md starts with a 0.1.1 baseline entry for everything released so far.

📝 Followup:

• Create the v0.1.1 tag on 72bc9e9, since the next changelog entry links back to it.

[pomek]

LGTM.