add: changesets + snapshot-this action

[zakariaelas]

What

• adds changesets to the monorepo
• adds a snapshot-this.yml action which will automatically create a snapshot release of any PRs whenever someone with write access to the repo comments "/snapshot-this" in the PR.

Why

• One of the best ways to completely move away from the old release workflow is to start adopting the changesets workflow slowly and incrementally. I think the better ways to do this could be through manually triggered automated workflows like this one. With this in mind, I thought it would be nice to try to solve a problem we've had in the past: publishing a PR to npm for faster development and a quicker feedback loop.

How

• How the snapshot-this gh workflow works: The workflow reacts to issue comments. More specifically, to comments having "/snapshot-this" as body. So given a PR which contains a changeset, when someone with write privileges to the medusajs repo comments /snapshot-this, the action will attempt a snapshot release of the current branch to NPM. Once that done, the action will comment on the PR with the published packages. Picture below captures this:

Note and next steps

• To test the action, snapshot-this.yml will need to be added to the main branch (which I think is master). We'll also need an NPM_TOKEN secret for publishing the snapshots to NPM. However, I am hoping to use this PR to kick off the discussion and get some opinions on the approach/strategy we'll use to move away from lerna and improve our release workflow.
• If this goes through, we'll also need to install the changesets github app which is a nice bot that, among other things, can remind you to include changesets in PRs. Thanks @pKorsholm for the reminder!

[pKorsholm]

Amazing as always Zak.
I love the snapshot-this command, just like with Kodiak it's a great step in the right direction without giving up too much control, and actually limiting the canaries to whatever pr's they are relevant for

One thing to note (not a comment to the code specifically), is the potential inclusion of the changesets github bot: https://github.com/apps/changeset-bot

This could help us enforce the use of changesets in the prs as well

[srindom]

LGTM! Let's get this in!