feat(init): detect existing package manager and auto-install packages

[fahslaj]

Description

Detects the package manager during lerna init by looking for existing lockfiles. Also makes the init command perform an installation task after the initialization to ensure Lerna is downloaded from the registry.

Motivation and Context

This makes it easier to add Lerna to a project that is managed by Yarn or Pnpm.

How Has This Been Tested?

This has been tested manually and covered by e2e tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (change that has absolutely no effect on users)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[JamesHenry]

Other than the one nit, please could update the docs to make note of this new behaviour?