Add `raw-private` (and possibly `raw-public` and `raw-secret`) key formats?

Currently, it's not possible to import or export a raw private X25519, Ed25519, ECDH or ECDSA key.

The spec says raw is "An unformatted sequence of bytes. Intended for secret keys." However, in the mentioned asymmetric algorithms it's actually used to import and export public keys. And unlike with pkcs8, spki and jwk, there's no natural way to tell from the format name and/or data whether it's a private or a public key.

So, we could introduce a raw-private key format for importing and exporting raw private keys.

Conversely, we could introduce raw-public for importing and exporting raw public keys, and deprecate the usage of raw for that purpose.

To complete the trio, we could introduce raw-secret for importing and exporting raw secret keys, and deprecate raw entirely.

(In the future, when we add ML-KEM, we may also want raw-seed to import/export the private key seed.)

Start with the linked Web Cryptography specification section for the raw key format and compare the private, public, and secret-key use cases described in this issue. Done means the project has a resolved decision about which additional formats, if any, should be introduced and how existing raw usage is handled.