feat: republish major tag on new npm version

[domharrington]

🚥 Fixes #9

🧰 Changes

GitHub Actions allows you to define a tag for an action to use that doesn't have to be a fully qualified semver version. This means we can publish v2.3.0 onto npm and then re-push that to v2 as a git tag. This means we dont have to prompt users to upgrade any time we push a non-breaking change. This is a pretty common pattern that GitHub does themselves, and we also do it in rdme too!

So then instead of telling people to do this:

- name: GitHub Action
  uses: readmeio/readme-micro@v2.3.0

We can just do this

- name: GitHub Action
  uses: readmeio/readme-micro@v2

🧬 QA & Testing

If you run this:

./bin/set-major-version-tag.js
$ git tag v2 -f -m 'Top-level tag pointing to 2.3.0'
Updated tag 'v2' (was fc99ec6)

You'll see how it works. We've automatically wired this up to the postversion event which is after the new npm version has been created and the new git tag is present.

[cvan]

to avoid needing executable permissions, this should be fine:

Suggested change

	"postversion": "rm -rf .old-version.txt && ./bin/set-major-version-tag.js"
	"postversion": "rm -rf .old-version.txt && node ./bin/set-major-version-tag.js"

since it already works, this should be fine