E2E tests with cypress (#802)

This PR:

* Adds end-to-end (E2E) tests using Cypress to cover critical features
in the WordPress playground.
* Adds these e2e tests to GitHub CI workflows for every PR

Depends on #801
Closes #765

The E2E tests cover:

- Switching PHP versions
- Switching WordPress versions  
- Networking support
- PHP extensions support
- Seamless mode
- OPFS synchronization in the browser
- "None" storage mode

The tests interact with the playground through:

- The Query API
- The user interface

This provides automated testing for the core functionality identified in
#765.

Additional tests will be tracked in follow-up issues:

- Connecting PlaygroundClient to the remote.html API
- OPFS synchronization with local filesystem  
- GitHub integration
- WordPress PR previewer
- Gutenberg PR previewer

<img width="2004" alt="CleanShot 2023-11-26 at 01 01 21@2x"
src="https://github.com/WordPress/wordpress-playground/assets/205419/e2353096-bbad-4fb9-a293-ea52fac14f05">

.github/workflows/ci.yml

@@ -21,13 +21,21 @@ jobs:
             - uses: ./.github/actions/prepare-playground
             - run: npx nx affected --target=lint
             - run: npx nx affected --target=typecheck
-    test:
+    test-unit:
         runs-on: ubuntu-latest
         needs: [lint-and-typecheck]
         steps:
             - uses: actions/checkout@v3
             - uses: ./.github/actions/prepare-playground
             - run: node --expose-gc node_modules/nx/bin/nx affected --target=test --configuration=ci
+    test-e2e:
+        runs-on: ubuntu-latest
+        needs: [lint-and-typecheck]
+        steps:
+            - uses: actions/checkout@v3
+            - uses: ./.github/actions/prepare-playground
+            - run: ./node_modules/.bin/cypress install --force
+            - run: npx nx affected --target=e2e --configuration=ci
     build:
         runs-on: ubuntu-latest
         needs: [lint-and-typecheck]
@@ -40,7 +48,7 @@ jobs:
     deploy_docs:
         if: github.ref == 'refs/heads/trunk' && github.event_name == 'push'
         # Add a dependency to the build job
-        needs: [test, build]
+        needs: [test-unit, test-e2e, build]
         name: 'Deploy doc site'
 
         # Grant GITHUB_TOKEN the permissions required to make a Pages deployment