jk test - jest without any setup

[dlespiau]

It'd be nice to start working towards integrating some common toolchain-things in jk itself. This is taking the nice properties of the go toolchain which integrates linting, formatting, testing, ... and doesn't need the user to do anything.

We could:

• embed jest in the standard library and call it from a jk test command
• ship a default config (for instance .*.test.js
• Allow overrides if the user wants something different, namely local config files take precedence over the embedded one.