Store key using `SubtleCrypto` API in browser

[joepio]

• We should use the importKey function with extractable set to false to provide more security.
• Support for ed25519 is not great, mostly behind flags.

[Polleps]

15% 😭. But we could future proof by checking support and conditionally using it while sticking to local storage for browsers that don't. Also we need to think about how we want to go about this, I believe WebCrypto can not return the private key, only sign data with it meaning @tomic/lib needs to use it directly. Right now storing the key is handled by browser because that is not really the concern of lib. WebCrypto might not be available in all contexts that lib should be able to run in. It is also not always appropriate for the key to be stored in general, for example on a server that has a store instance for each connected user.