feat(ci): test JS compilation of the engine

.github/workflows/engine_js_build.yml

@@ -0,0 +1,18 @@
+name: Test JS build
+
+on:
+  pull_request:
+  merge_group:
+  workflow_dispatch:
+  push:
+    branches: [main]
+
+jobs:
+  engine-js-build:
+    if: ${{ github.event_name == 'workflow_dispatch' || github.event_name == 'merge_group' }}
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: DeterminateSystems/nix-installer-action@main
+    - uses: DeterminateSystems/magic-nix-cache-action@main
+    - run: nix build .\#hax-engine.passthru.js -L  

CI.md

@@ -17,7 +17,9 @@
    FStar, we do not run Coq or FStar on the extractions.
  - [`test_installs`](./.github/workflows/test_installs.yml): compiles
    the toolchain on two versions of Ubuntu and two versions of MacOS
-   using `apt` or `homebrew` and the `setup.sh` script.
+   using `apt` or `homebrew` and the `setup.sh` script;
+ - [`engine-js-build`](./.github/workflows/engine_js_build.yml): tests
+   the build the JS version of the engine.
 
 ## Merge queue
 Additional actions are triggered on pull requests in the merge queue. They are