Reject JWK keys that have conflicting "use" and "key_ops"

[jonasfj]

In chromium this is implemented in VerifyUsages: https://source.chromium.org/chromium/chromium/src/+/main:components/webcrypto/jwk.cc;l=144

Something similar should be done for all the places we import JWK keys.

Then we should still strip use and key_ops before importing keys on the web, unless we decide to properly implement capabilities, see https://github.com/google/webcrypto.dart/issues/53