feat(init): Create .gitignore with lerna init

[fahslaj]

Makes the lerna init command now generate a .gitignore file that excludes node_modules/ by default. Existing .gitignore files are untouched.

Motivation and Context

All lerna workspaces will have a node_modules folder that should be excluded from git, so this saves the developer the step of creating the .gitignore file themselves.

How Has This Been Tested?

This has been tested manually, as well as with newly updated unit, integration, and 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.