ci: add `previews.yml` to re-generate assets

[sgoudham]

This PR comes from my frustration on the lack of good font rendering on Linux.

I was trying to re-generate the previews to get them to match the existing previews. Unfortunately, Puppeteer seems to struggle with consistency across operating systems as highlighted in https://github.com/puppeteer/puppeteer/issues/661

I tried the solutions commented in the above issue but nothing seemed to work. Ultimately, I gave up and decided to try and see if we can make it a concern of the CI. I was finally successful and this PR adds a new workflow previews.yml that re-generates the individual flavour previews and the catwalk preview.

You can view the output of a successful run in https://github.com/sgoudham/ci-playground/pull/10

Limitations / Concerns:

• We're using the macos-latest runner which is basically the entire reason why the font rendering matches up with the existing previews.
• Telling pnpm to cache the directory results in an error where the chrome executable cannot be found. I tried setting the cache directory in the Puppeteer config file but it still failed and, honestly, I gave up after that.
• The pnpm scripts are not correctly outputting exit code 1 and failing the CI pipeline. I'm not too fussed about it since we know if it's failed if it's not raised a PR when we're expecting it to create one but this should be looked into.