Fix prepack for npm-only publish CI

The publish workflow installs deps with npm, so prepack must not call
`pnpm run`. Switch both inner script calls to `npm run`, which works
under both npm and pnpm. Also add `tsx` as a direct devDependency — it
was previously only an optional peer of vitest, so `npm install` would
not fetch it and the generate-ast-types step would fail.

Caught by the v1.0.0 release run: npm publish failed in prepack with
`sh: 1: pnpm: not found` after the restructure rewired the script.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Peter Stenger