Convert to module

[markhughes]

This is the "mega update" to make droppy more maintainable, as I've had a lot of struggles myself keep it up to date due to its old syntax.

Changes include:

• Converting to new modules syntax
• Moved to Node v20
• Move to new ESLint ecosystem

Todo:

• Convert client to module
• Give client its own a build step, so we can use imports, perhaps with webpack so we can use treeshaking, etc.

What are the changes and their implications?

Checklist

• [ ] Changes covered by tests (tests added if needed)
• [ ] PR submitted to droppyjs.com for any user facing changes