Switch to capnp-es

[penalosa]

The capnp-ts dependency we use for Miniflare (and for type generation in workerd for constructing and reading the capnp format for the runtime) is blocking workerd upgrading TS versions to 5.7 (and will block Miniflare when we get around to upgrading—we're currently on 5.6). capnp-ts effectively abandoned and hasn't been touched in 4 years, so this PR switches us to use capnp-es. It's a followup to unjs/capnp-es#10.

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• Public documentation
  • TODO (before merge)
  • Cloudflare docs PR(s):
  • Documentation not necessary because: internal packaging change

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