introduces ca-changesets plugin

[t-monaghan]

A thin wrapper to @changesets/cli, using the npx command to call changeset.
This will install changeset if it does not already exist, without requirement of a package.json file, and without creating a node_modules/ directory.

I have ensured that:

• This plugin will install nodejs, however the consuming devbox project's node version will not be clobbered, as the project's environment is loaded after the plugin environment.

[elliotthilaire-ca]

Nice work. Especially the extra thinking around checking if the nodejs version would be clobbered. And working out how to get it installed without the node_modules folder.

I tested on ca-hellowworld-ruby by adding:

"include": [
    "github:cultureamp/devbox-extras/tfm/changeset-thin-fef-1768/?dir=plugins/ca-changesets"
  ],

It looks like changeset wasn't appearing on my path so I couldn't call it with just changeset

I was able to call it directly with:

.devbox/virtenv/cultureamp.devbox-extras.ca-changesets/bin/changeset

and verified that that works.

[t-monaghan]

I've fixed the issue with changeset not appearing on path. It's due to a bug in devbox not being able to manipulate an environment variable twice. See: jetify-com/devbox#2138

[elliotthilaire-ca]

Nice. Have tested against ca-helloworld-ruby and this works so I think you can merge.

(It didn't seem to work first go. I tried deleting .devbox and it still didn't seem to work. after a devbox update it worked though. THere's no mention of the plugin in devbox.lock so I'm not sure how that works.)