Export feature for P-521 and X25519 cryptoKey

[Srabutdotcom]

It would be better if Deno fully support for Webcrypto API,..

I have tested that deno can create P-521 and X25519 cryptoKey like a charm.
P-521 cryptoKey

• export to JWK failed on both private and public key - NotSupportedError: Unsupported namedCurve
• export to RAW failed - TypeError: expected valid private EC key

X25519 cryptoKey

• export to JWK failed on private key only - NotSupportedError: Not implemented

[hertzg]

The X25519 private key export as jwk works in latest node LTS and does not issue experimental warning anymore.
This is useful when one wants to "derive" a public key from a private key. When exporting to JWK the d field would contain the base64url encoded privateKey and the x field - publicKey.

Sample code to reproduce:

// node.mjs contents
const { privateKey } = await crypto.subtle.generateKey(
  { name: "X25519" },
  true,
  ["deriveKey", "deriveBits"]
);
const jwk = await crypto.subtle.exportKey("jwk", privateKey);
console.log(jwk);

When running under node v22.13.0:

$ node -v     
v22.13.0
$ node node.mjs
{
  key_ops: [ 'deriveKey', 'deriveBits' ],
  ext: true,
  crv: 'X25519',
  d: '2LAP7t3KsI7n2JxKpatigiSxFS8IcNInjTnn-Bxx6VE',
  x: 'ldy_Ujc9T1Ml4zwyjVKdffGMiYXJJ4QTtn0P6VA7_H4',
  kty: 'OKP'
}

When running under deno 2.1.5:

$ deno -v
deno 2.1.5
$ deno node.mjs 
error: Uncaught (in promise) NotSupportedError: Not implemented
const jwk = await crypto.subtle.exportKey("jwk", privateKey);
                                ^
    at exportKeyX25519 (ext:deno_crypto/00_crypto.js:4536:15)
    at SubtleCrypto.exportKey (ext:deno_crypto/00_crypto.js:1054:18)
    at file:///Users/hertzg/projects/wireguard-tools/node.mjs:6:33