Add a lerna/monorepo switch or alternate cookiecutter

[bollwyvl]

Problem

It doesn't take long for an extension to benefit from multiple, separately npm-packaged extensions which are still distributed in the same pypi package. This is particularly relevant to themes.

Proposed Solution

One of:

• add a use_lerna switch to this repo
• split out an extension-cookiecutter-ts-lerna, as it's a whole other thing

Additional context

This stemmed from a discussion on theme extensions, which don't play well outside of strict light/dark variants, but the monorepo pattern

• adds lerna to package.json
• uses the packages/ directory layout
• update pyproject.toml to ensure the static assets are happy
• uses a composite build

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