Use stdio log prefixing for process.stdio

[guybedford]

This implements an alternative to #4965 that outputs to console.log with the prefix strings stderr: and stdout: instead, along with a new configuration option:

struct LoggingOptions {
  structuredLogging @0 :Bool = false;
  # Override of top-level structured logging (only when true).
  # If true, logs will be emitted as JSON for structured logging.
  # When false, logs use the traditional human-readable format.
  # This affects the format of logs from KJ_LOG and exception reporting as well as js logs.
  # This won't work for logs coming from service worker syntax workers with the old module registry.

  stdoutPrefix @1 :Text = "stdout:";
  # Prefix process.stdout output with "stdout: "
  # Can be set to the empty string or a custom prefix to customize.

  stderrPrefix @2 :Text = "stderr:";
  # Prefix process.stderr output with "stderr: "
  # Can be set to the empty string or a custom prefix to customize.
}

which can be used to both customize and disable prefixing.

[jasnell]

Few nits, but generally LGTM

[petebacondarwin]

Tried this out locally with Wrangler and the output looks good to me.

Only tiny, non-blocking, thought is whether the configuration option could be to specify the prefixes, rather than just a boolean. This would make it really nice for people to be able to still have prefixes but avoid collisions with other prefixes.

[github-actions]

The generated output of @cloudflare/workers-types matches the snapshot in types/generated-snapshot 🎉

[jasnell]

LGTM with one small nit on the config capnp

[petebacondarwin]

We'll still need to modify the Miniflare tests to account for the prefix. I can't put up a PR to relax those tests for now.

Actually since we are no longer changing how console.debug() is passed to stdout, these tests should not need to change.