Adapt to React api: add context argument to compat/Children map api

[ParSal123]

React's children api map function supports a "context" argument that we don't. Fixes https://github.com/preactjs/preact/issues/2644.

Based mostly on #2645.

There was a problem though: When I ran npm run test:karma, it didn't test Children.test.js. Even the pipeline on the forked project didn't run Children.test.js. I couldn't figure it out. Don't know if this is an issue or just misconfiguration.