Fix internal library export hygiene

[m0ar]

There are some best-practices we should be following in our internal libraries:

1. No reliance on barrel files (prevents tree-shaking)
2. Pick one export methodology to replace the barrel file:
   • actual API in index.ts (more verbose, singular namespace, less dependent on module structure)
   • package.json export maps (less verbose, isolated namespace, more responsibility on module structure)
   • combination (see notes below)
3. Ideally set sideEffects: false in package.json (tells bundlers that the import is pure)
4. Explicitly export type where possible
5. Only named exports (no export default, which can break bundler analysis in certain cases)
6. No dynamic imports (don't think we have this in any lib atm)

Checklist:

• @desci-labs/nodes-lib
• @desci-labs/desci-codex-lib
• @desci-labs/desci-contracts:
  • Can we drop entirely from webapp?
• @desci-labs/desci-models

Notes on export methodologies:

• Explicitly preventing import of "package-private" internals directly from /dist requires we use export maps
• We can use a hybrid approach, by putting most common API items in a root-exported index.ts, and use export maps to expose module internals