Consider adding getErrorCode

[aleclarson]

We already have getErrorMessage for safely reading a message from an unknown thrown value. A matching getErrorCode could be useful for errors from Node, HTTP clients, SDKs, and custom domain code that expose a code field.

Proposed shape:

getErrorCode(error: unknown): string | undefined

Proposed behavior:

• Return error.code when it is a non-empty string.
• Return undefined when there is no usable code.
• Do not fall back to name, status, or a generic value.

Example:

try {
  await sendRequest()
} catch (error) {
  const code = getErrorCode(error)

  if (code === "ETIMEDOUT") {
    // handle timeout
  }
}

Open question: should numeric codes be stringified, returned as-is, or ignored?

Any objections to adding this?