Optional peer dep installation

[acolytec3]

In working on #3188, @jochem-brouwer and I discovered that if we delete and update package-lock.json in the monorepo root in order to clean up missing or misaligned dependency versions between master and a PR branch, the peer dependencies in package.json marked with optional: true are not automatically installed. This is intended behaviour, based on past discussion from when we switched to vitest because we didn't want to require users of our libraries to have to install large browser binaries necessary for running our browser tests. That said, it's had the unintended consequence of causing headaches when updating package-lock.json because npm i won't install these dependencies automatically which leads to the browser tests failing (and the related CI job).

So, I'm proposing one of two alternatives:

1. We return the three deps in question (@vitest/browser, webdriverio, and playwright) to the devDependencies so they are always installed unless the user runs npm i --omit=dev. This is certainly the easiest approach since the browser binaries are generally needed for testing and are really part of the development dependency set (though admittedly we don't think about the browser tests much).
2. We could add a new npm install browser-deps script that manually installs these three dependencies. If we go this route, we need to make sure the CI job for the browser tests runs this script. The tricky bit here is that we would need to remember to run this when we update package-lock.json

I'm not sure if I have a strong preference here but maybe variant 2 is the cleanest (and limits when these extra deps get installed since they would be skipped in the other CI runs)

[holgerd77]

Thanks for the detailed write-up. Yes, agree, would also have a tendency for a solution in the line of variant 2. 👍

[acolytec3]

Option 2 should be addressed in #3188