The existence of pnpm-lock.yaml should not prevent npm users

What problem does this feature solve?

Currently "packages/@vue/cli-shared-utils/lib/env.js" - checkPnpm() function throws error "Error: the project seems to require pnpm but it's not installed." if project contains "pnpm-lock.yaml" file while not installing pnpm in modules I think pnpm-lock.yaml file offers a choice rather than telling the project is "pnpm only". Many open-source projects(element-plus) have pnpm-lock.yaml files. but user may still choose npm as package manager. Anyway, this should be a warning instead of an error which prevent project serve.

What does the proposed API look like?

Change throwing error to warning