Moved the SpiderMonkey-intensive parts to `UniFFIJsBridge`. My hope here is that UniFFI engineers can update the code to response to FFI layer changes, without having to learn a ton about the JS layer -- and vice-versa for DOM engineers. Couple other smaller changes: * Updated the UniFFICall code to use "Lift" and "Lower". I'd like to start leaning into these terms more, since their useful and devs already understand them. I think I avoided this because it means that values get lifted multiple times, but I don't think that's hard to understand. * Removed module descriptions from the README. I'd rather put docstrings in the source files, since there's less chance of those getting stale. Differential Revision: https://phabricator.services.mozilla.com/D240697
uniffi-js
This directory contains C++ helper code for the UniFFI Rust library (https://github.com/mozilla/uniffi-rs/).