CLI: Use closest lockfile to determine package manager

[IanVS]

Issue: #20463

What I did

Sometimes people have a yarn.lock file in their home directory accidentally, and if they do, Storybook CLI uses yarn, even if their package is using pnpm and has a pnpm lock file.

With this change, instead of looking for yarn.lock and pnpm-lock.yaml independently, we look for both at the same time, and find-up returns the closest one (falling back to yarn if there are multiple in the same directory).

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

[shilman]

Nice one @IanVS -- appreciate the nice tests!!! 💯 Happy new year!