Add changesets

[webpro]

Follow-up to #612 and targeting that branch.

This PR introduces Changesets to handle our changelog and package publishing needs. Please read some of the docs to get an idea of what it is all about. Here's a an article with a quick rundown: Ditching manual releases with Changesets.

Also see #612 + comments for additional context. This PR contains:

• Changelog CLI that writes .changesets/*.md files (the equivalent of common/changes/**/*.json in Rush projects).
• An initial GitHub Action that creates a pull request with all changes in the main branch.
  • The PR will be updated until we're ready to release.
  • When we're ready, we merge this PR and it will publish the (non-private) packages to npm.
  • We need to install GITHUB_TOKEN and NPM_TOKEN.
  • This PR includes the first workflow (see below); probably good to start with the third.
• A (temporary) script to convert the Rush JSON files to the Changesets Markdown format if/when necessary.

Three workflows:

1. Use GitHub Action w/ available automation (versioning + publishing), in this PR.
2. Use this GitHub Action only for versioning. Publishing manually (using Changesets CLI).
3. Omit the Action, and use the Changesets CLI to do both steps manually.

When we open new pull requests we can add a changeset (the equivalent of rush change):

pnpm changeset

After merging this PR we can install the Changesets bot which comments on PRs that don't contain a changelog (the equivalent of rush change --verify). So with normal settings developers will be notified in the PR itself + email.

Extra scripts

Convert Rush changelog (common/changes/**/*.json) to Changesets (.changesets/*.md) format:

npx tsx packages/tools/scripts/convert-changelog.ts

Generate packages.json in root:

npx tsx packages/tools/scripts/generate-packages-json.ts

Publish one, set, or all packages (from root):

pnpm publish-set

Notes

• Both Rush and Changesets aim to publish everything at once, and publishing a single or subset of packages is not (yet?) possible in a fully automated fashion. We can experiment with using the ignore option to exclude "everything else" from being versioned and published.
• Changesets updates CHANGELOG.md in each package. We can remove the CHANGELOG.json files as we go.

Preview

To get an idea of what publishing looks like, here's a screenshot with the current settings:

The pnpm publish-set is a script that basically adjusts the ignore: [] config of Changesets in case we don't want to publish everything. Run this script which will populate the config: [] with everything we won't publish (yes it's a hack) and runs the regular Changesets version + publish flow afterwards:

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