doc: example generating keypairs with WebCrypto & node:crypto

Start from the JavaScript example in the issue, especially generateKeyPair, generateSecp256k1KeyPair, and main, and compare its WebCrypto and node:crypto usage. Confirm the documented algorithm names and expected generated files, including the open question about secp256k1 support in WebCrypto; done means the example clearly covers the intended keypair generation.

'use strict';

let Fs = require('node:fs/promises');

async function generateKeyPair(algo) {
  let keyPair = await crypto.subtle.generateKey(algo, true, ['sign', 'verify']);

  let name = algo.namedCurve || algo.name;

  let privateKeyAb = await crypto.subtle.exportKey('pkcs8', keyPair.privateKey);
  let privateKeyDER = new Uint8Array(privateKeyAb);
  let privateKeyPath = `./key.${name}.der`.toLowerCase();
  await Fs.writeFile(privateKeyPath, privateKeyDER);

  let publicKeyAb = await crypto.subtle.exportKey('spki', keyPair.publicKey);
  let publicKeyDER = new Uint8Array(publicKeyAb);
  let publicKeyPath = `./pub.${name}.der`.toLowerCase();
  await Fs.writeFile(publicKeyPath, publicKeyDER);

  console.info(`Key pair generated and saved as ${privateKeyPath} and ${publicKeyPath}`);
}

let Crypto = require('node:crypto');

async function generateSecp256k1KeyPair() {
  return new Promise(function (resolve, reject) {
    Crypto.generateKeyPair(
      'ec',
      {
        namedCurve: 'secp256k1',
        publicKeyEncoding: {
          type: 'spki',
          format: 'der',
        },
        privateKeyEncoding: {
          type: 'pkcs8',
          format: 'der',
        },
      },
      async function (err, publicKey, privateKey) {
        if (err) {
          return reject(err);
        }

        await Fs.writeFile('./key.secp256k1.der', privateKey);
        await Fs.writeFile('./pub.secp256k1.der', publicKey);

        console.info(
          'secp256k1 key pair generated and saved as ./key.secp256k1.der and ./pub.secp256k1.der',
        );
      },
    );
  });
}

async function main() {
  await generateKeyPair({ name: 'Ed25519' });
  await generateKeyPair({ name: 'ECDSA', namedCurve: 'P-256' });
  // this should be possible with WebCrypto as well, but I don't know the name to use
  //await generateKeyPair({ name: 'ECDSA', namedCurve: 'secp256k1' });
  // it does work with node:crypto
  await generateSecp256k1KeyPair();
}

main().catch(function (err) {
  console.error(err.stack);
  process.exit(1);
});