Feature: Support pnpm as alternative package manager to yarn

Feature description

Offer support for the existing "packageManager" configuration setting for Node projects.

Please consider removing the explicit dependency on yarn in the "backstage-cli" and support alternative package managers like pnpm.

Context

We have a monorepo built with a lot of pnpm automation, but the Backstage instance directories including all projects / workspaces require yarn and must be excluded with special treatment.

pnpm can offer some benefits to teams over yarn depending on the features they value most.

✌️ Possible Implementation

Read the preferred package manager tool from the "package.json" file and the configuration setting "packageManager". Support this tool in all automation including "backstage-cli".

Have you spent some time to check if this feature request has been raised before?

• I checked and didn't find similar issue

Have you read the Code of Conduct?

• I have read the Code of Conduct

Are you willing to submit PR?

No, but I'm happy to collaborate on a PR with someone else