Core install? (integrated, but avoid puppeteer install)

**TLDR**: I want to have `backstopjs` in my project's package.json, but still be able to avoid the chromium download that usually happens (via puppeteer) on install.

Standard install of backstop with `yarn add backstopjs` will include the puppeteer installation, and that will in turn include the chromium download. On a CI system (or in certain corporate setups) this will fail because of the direct download.
The workaround ("Set "PUPPETEER_SKIP_CHROMIUM_DOWNLOAD" env variable to skip download.") is fine for a one-off developer setup, and fine if you don't need to have backstopjs integrated with the project.

I'm trying to find a way to integrate backstop into a project (shared among various developers on Mac and Windows, and always behind the corporate wall), without requiring a bunch of manual setup for each person.

Basically, I can't just add `backstopjs` to the `package.json` `devDependencies` because it will break the yarn install for the whole project. And if I instead provide instructions (or an npm script) for the global install of backstopjs (with the SKIP...DOWNLOAD environment), then I can't use backstop in an [integrated way](https://github.com/garris/BackstopJS#integration-options-local-install).

```json
scripts: {
"backstop:install": "cross-env PUPPETEER_SKIP_CHROMIUM_DOWNLOAD=true yarn global add backstopjs",
}
```
*Best solution I have, but doesn't allow for project integrations*

I noticed that puppeteer has a default install, but they also have a [puppeteer-core](https://www.npmjs.com/package/puppeteer-core) install that skips the chromium download. Would it be possible to get a `backstop-core` package that works similarly? Or can anyone recommend any other way to handle this? - Thanks

Start by reviewing the project's package.json dependency setup and the documented local-install integration options. Compare the requested project-local install with the PUPPETEER_SKIP_CHROMIUM_DOWNLOAD workaround and puppeteer-core alternative; done means a project can depend on BackstopJS without an unwanted Chromium download while retaining integration.

Written by the indexing model from the issue text.