Updated the uniffi-bindgen-gecko-js code to use two things I've been
experimenting with:
* Use the IR pipeline code to generate the structs used to render the
templates.
* A new test fixture for bindings generators. This one targets bindings
generators specifically, it doesn't try test the scaffolding code and
it's not based on real-world example code. I originally thought it
would be a single crate, but I ended up needed 2 in order to test
external types. (https://bugzilla.mozilla.org/show_bug.cgi?id=1948961)
Differential Revision: https://phabricator.services.mozilla.com/D242385
90368cc237