[cli] Support installing dependencies with yarn

Provide a way for new projects to use yarn over npm for installing dependencies.

Use Case

I primarily use yarn for all my projects, so when starting new cdk projects I have to delete the package.lock and node modules and reinstall using yarn.

Proposed Solution

It seems like the project by default uses yarn in its codebase, so maybe default to using yarn if it's there, and if not use npm.

Another option is to provide a flag on the init command that will indicate if to use yarn.

Other

I'd like to get feedback on the approach before I start a PR for this.

• :wave: I may be able to implement this feature request
• :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request