Manage code duplication

[rviscomi]

The core capo logic is effectively duplicated across the snippet, WPT custom metric, BQ function, and now the Chrome extension.

Devise a way to have all of the core logic defined in one place and integrated into each surface. This will probably require a build process.

[rviscomi]

Made progress in #45 using ESM to break up the capo logic into reusable scripts for both the snippet and extension.

TODO for this issue:

• [ ] generate WPT script from reusable scripts
• [ ] generate BigQuery SQL from reusable scripts