devx: Create + document a release checklist

Motivation

For maintainers , we can make it clearer what to do each time we cut a new release as part of our contribution guidelines. I haven't cut a new release of Vega's core package, so when I try it for the first time, I can have a go at this .

References

• NPM publishing guide for single packages: https://github.com/vega/vega/discussions/4171
• @rosstitmarsh had the great idea of including marking unreleased functions as 'released' with the current version as part of this in #4280 , and I'll make note of anything else that
• I should probably interview @lsh to see if I'm missing anything else
• Use Github "milestones" feature to help plan what features are going into a release