Migrate sub-packages to package.json exports field

[vitoke]

Currently sub-packages like @rimbu/list/custom are exported using folders containing their own package.json. However, it is now possible to use the exports field in package.json to do this. All rimbu packages should therefore be migrated to this new structure.

This also allows adding conditionals for various contexts like imports and require. This could also potentially add support for bun, since it seems to only run on the TypeScript code, and not on the transpiled code.