Generate NPM tag within publish-release bash script

[vanitabarrett]

Part of #2004

What

Add logic to existing publish-release bash script to work out the correct NPM tag for a release before publishing it.
This also adds the option to provide a tag yourself if the suggested one doesn't suit.

Why

If we want to release versions of GOV.UK Frontend older than the current major version, e.g: releasing a version 4.5.0. when we've already released version 5.0.0, then we need to be able to override NPM's default "tag" which tags the most recent release as "latest".

Tagging v4.5.0 as "latest" would mean people doing npm install govuk-frontend would get v4.5.0 instead of v5.0.0.

Unfortunately, the only way to NOT tag something as "latest" is to provide a different tag to tag it with. This script compares the tags on Github with the version being proposed. If the version number being proposed is smaller than the highest version tag on Github, the NPM tag will be latest-[MAJOR-VERSION-NUMBER] e.g: latest-4

Examples

Current release	New release	Tag
4.1.0	4.1.1	latest
4.1.0	4.2.0	latest
4.1.0	3.14.0	latest-3
4.1.0	5.0.0	latest
5.0.0	4.1.5	latest-4

[36degrees]

This is really neat – great idea 🙌🏻

At the risk of bike-shedding, I think e.g. v4-latest might be clearer than latest-4… although having said that it looks like bootstrap do something very similar so maybe there's a convention here I'm not aware of…

[lfdebrux]

At the risk of bike-shedding, I think e.g. v4-latest might be clearer than latest-4… although having said that it looks like bootstrap do something very similar so maybe there's a convention here I'm not aware of…

I don't think npm will let you use v4-latest, as they say a tag can't be a valid semantic version, and they say 'the simplest way to avoid semver problems with tags is to use tags that do not begin with a number or the letter v.' [1].

[vanitabarrett]

This is really neat – great idea 🙌🏻

At the risk of bike-shedding, I think e.g. v4-latest might be clearer than latest-4… although having said that it looks like bootstrap do something very similar so maybe there's a convention here I'm not aware of…

This was my first though, but what @lfdebrux said above - looks like npm advise against doing this. We could do latest-v4 if that's any better 🤷

[36degrees]

Ahah! That all makes sense. Personally I think latest-v4 makes it a little clearer what the number represents, but not precious about if we'd rather just follow what bootstrap are doing.

[lfdebrux]

One small stylistic suggestion, otherwise looks good to me!