Add detecting "secp256k1" as a crv type for compatibility

Change this [this line](https://github.com/PeculiarVentures/webcrypto-liner/blob/7952178b88897d846e1e385e34036bcd052574c9/src/mechs/ec/crypto.ts#L122) to include detecting the string "secp256k1".  Required for use with other libraries like [Jose](https://github.com/panva/jose) that only accept that string as `crv` type in JWK.