Bug 1955298 - uniffi-bindgen-gecko-js: start using the IR pipeline, r=markh,frontend-codestyle-reviewers

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

Cargo.toml

@@ -65,6 +65,7 @@ rust-version = "1.82.0"
 # Shared across multiple UniFFI consumers.
 uniffi = "0.29.2"
 uniffi_bindgen = "0.29.2"
+uniffi_pipeline = "0.29.2"
 # Shared across multiple application-services consumers.
 rusqlite = "0.33.0"
 # Shared across multiple glean consumers.