Fix published utils export

[jaasieldelgado131]

Summary

• point the published astro-portabletext/utils export at generated JavaScript instead of the TypeScript source file
• add a package prepack build that emits the JS used by that subpath
• update internal relative imports so the emitted ESM resolves in Node

Reproduction

With the published astro-portabletext@0.13.0 package:

mkdir repro && cd repro
npm init -y
npm install astro-portabletext@0.13.0
node --input-type=module -e "await import('astro-portabletext/utils')"

Node fails with ERR_UNSUPPORTED_NODE_MODULES_TYPE_STRIPPING because the subpath resolves to lib/utils.ts under node_modules.

After this change, npm pack runs the package build and the packed tarball contains lib/utils.js, lib/context.js, and lib/internal.js; installing that tarball in a clean project allows import('astro-portabletext/utils') to succeed.

Validation

• corepack pnpm --dir astro-portabletext build
• corepack pnpm --dir astro-portabletext check
• corepack pnpm --dir lab check
• corepack pnpm --dir lab exec uvu -r tsm src/lib
• corepack pnpm --dir lab build:fixture
• corepack pnpm --dir lab exec uvu src/test .test.js
• corepack pnpm lint
• npm pack --json --pack-destination <temp> from astro-portabletext/, then clean install of the tarball and import('astro-portabletext/utils')

Note: pnpm test:ci is a shell wrapper (./scripts/ci.sh) and does not run directly in this Windows environment, so I ran the same underlying commands individually.