refactor: create monorepo w/ lerna

[dkundel]

This PR is the first step in the process of turning all parts of the Serverless Toolkit into a monorepo as discussed in #20

The next step after this would be to import the respective other packages. Those will all come through the lerna import tool that will replay the git history so we have the old history.

I'll open separate PRs for each of those once this one is merged.

I'm also doing all the changes on the main branch to retire the master branch and eventually delete it.

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