Add w3c_webcrypto support

[sumo]

I am trying to add webcrypto support but I cannot get the project to build with bazel - will raise another issue.

I have attached the patch that (I think) adds webcrypto. Hopefully it will be quicker for you to apply and release rather than me to get my build issue fixed.
0001-Attempt-to-add-web-crypto.patch.txt

[realityforge]

It looks like there are a few things still need to be addressed in this patch. I would recommend getting Bazel building and then look at the CONTRIBUTING.md document in the sections "Providing a specific numeric type for a JavaScript API" and "Providing parameter names". I tend to go through a few rounds of generating the API, clean up integers/parameter names. Potentially cleanup externs in closure compiler to add more useful typing. And when thats ready get a release going.

I know the j2cl/elemental2 team is trying to get an elemental2 release happening atm but if you miss that round I tend to release versions to maven central when there is demand.