Set up infrastructure for Jest (JavaScript) tests (Qiskit#282)

Prework for Qiskit#208.

Abby and I decided that we want Jest tests for two purposes:

1. Checking that web components are in sync between the Pytorch and Furo
themes, e.g. we didn't forget to update one of the folders.
1. We could do this either by using Snapshot testing or by simply
checking that the file contents are equal.
2. We have some non-trivial logic in
Qiskit#267 and want to write
automated unit tests.

This PR adds the basic infrastructure, like setting up CI to install
Node.js. That will make it easier to review follow-up PRs that add
actual tests.

> We could do this either by using snapshot testing

I think we'd benefit from adding snapshot testing for the top nav bar
rendering correctly. I want to do this in a follow up PR so we can
properly decide if we want to add snapshots or not; the decision to use
snapshots is separate from whether to use Jest generally.

.github/workflows/main.yml

@@ -9,16 +9,24 @@ jobs:
       - uses: actions/checkout@v2
         with:
           fetch-depth: 0
-      - name: Set up Python 3.9
+      - name: Set up Node.js
+        uses: actions/setup-node@v2
+        with:
+          node-version: 18
+      - name: Install Node.js dependencies
+        run: npm ci
+      - name: Run Node.js tests
+        run: npm test
+      - name: Set up Python
         uses: actions/setup-python@v2
         with:
           python-version: 3.9
-      - name: Install ubuntu Deps
+      - name: Install Ubuntu deps
         run: sudo apt-get install -y pandoc graphviz
       - name: Install tox
         run: |
           python -m pip install -U tox
-      - name: Build Docs
+      - name: Build example docs
         run: tox run -e py
       - name: Compress Artifacts
         run: |

.gitignore

@@ -130,3 +130,6 @@ dmypy.json
 # Pyre type checker
 .pyre/
 .DS_Store
+
+# JavaScript
+/node_modules

CONTRIBUTING.md

@@ -36,6 +36,18 @@ This subfolder contains some example `.rst` files that show how to implement spe
 3. Run `pip install -e .`
 4. Run `sphinx-build -b html example_docs/docs/ example_docs/docs/_build/html` to build the documentation. This will create the `example_docs/docs/_build` subfolder where you can see the html files generated based on the `.rst` files in the `docs` folder and the styles pulled from the `qiskit_sphinx_theme`.
 
+------
+## Run JavaScript tests
+
+We write some tests in JavaScript (Node.js) to have automated checks of the theme, e.g. that certain components render properly.
+
+To run these tests, you first need to install Node.js on your machine. If you expect to use JavaScript in other projects, we recommend using [NVM](https://github.com/nvm-sh/nvm). Otherwise, consider using [Homebrew](https://formulae.brew.sh/formula/node) or installing [Node.js directly](https://nodejs.org/en).
+
+Then:
+
+1. `npm install`
+2. `npm test`
+
 ------
 ## Updating bundled web components
 

__tests__/utils.test.js

@@ -0,0 +1,5 @@
+const { expect, test } = require("@jest/globals");
+
+test("dummy test to check Jest infrastructure works. Remove after", () => {
+  expect(true).toBe(true);
+});