fix: detect package manager, fallback to npm.

[malewis5]

Summary

• Update CLI to detect package manager for package.json.
• Add test to verify new detect function.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @malewis5 to sign the Salesforce Inc. Contributor License Agreement.

[zimeg]

📫 In discussion at #170!