Use separate jobs for JVM/JS/Native?

This is another good idea h/t @bblfish.

The reason for this change is that the matrix axes for the different platforms are fundamentally different:

• JVM projects generally want to test different on different JVMs and occasionally different OSes
• JS projects only need one JVM, but then there is Node.js, browsers, etc. to test against
• Native projects also only need one JVM, but I presume that testing again different OSes may be a bigger deal here

Furthermore, these platforms require different setups e.g. installing Node.js or browsers or native libraries.

Currently we use a single job with a matrix with axes for each of these things. Then we add a bunch of exclusions and conditional steps to remove all the combinations that don't make sense. But we still end up with confusingly-named jobs e.g. a JVM job that lists Node.js as its JS runtime because that is the value in the matrix even though its obviously unused.

Doing this right will be a fairly non-trivial change, so I have to think more about it. But I do think it has good potential make cross-platform builds nicer and easier to work with.

The issue names no files or tests. Start by reviewing the current CI matrix, its exclusions, and the platform-specific setup steps; done means separate JVM, JS, and Native jobs with platform-appropriate matrix axes, setup, and job names.