chore: Update planx-core to ESM module

[DafyddLlyr]

What does this PR do?

• Converts this project from CommonJS to ESM
• Associated changes -
  • Convert from jest to vitest
  • Convert from ts-node to tsx
  • Convert from lodash to lodash-es
  • Add type assertions to JSON imports
  • Fix imports to use .js in filenames

Some of these changes were a bit haphazard - one following the other as I hit issues. If anything is too unclear / too big, please shout and I'll work on splitting this up.

Why?

• Required for feat: Import schema from library #600 (import ESM schema lib)
• Similarly, this might be needed if we decide to publish this repo as a package
• ESM is the way things are headed - worth making the change and not publishing in CommonJS!

[jessicamcinchak]

Thanks for this !! Dense changes indeed but all looks good here - and corresponding planx-new PR/pizza seems healthy 🎉

[freemvmt]

Looks great! Brings up happy mems of doing the same for the API 😅

My only question is whether there is some way to simplify the tsconfig setup? e.g. is one an extension of another. and can be represented as such? could the base tsconfig.json replace any of the particular tsconfig.x.json files? haven't looked at it closely, but just a thought!