Add Cypress E2E testing framework

.github/workflows/e2e.yml

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+name: E2E Cypress Tests
+
+on:
+  push:
+    branches:
+      - develop
+      - main
+  pull_request:
+    branches:
+      - develop
+
+jobs:
+  cypress:
+    name: ${{ matrix.core.name }}
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        core:
+          - {name: 'WP latest', version: 'latest'}
+          - {name: 'WP minimum', version: 'WordPress/WordPress#6.0'}
+          - {name: 'WP trunk', version: 'WordPress/WordPress#master'}
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - uses: actions/setup-node@v4
+      with:
+        node-version-file: '.nvmrc'
+
+    - name: Install dependencies
+      run: npm ci
+
+    - name: Set the core version
+      run: ./tests/bin/set-core-version.js ${{ matrix.core.version }}
+
+    - name: Set up WP environment
+      run: npm run env:start
+
+    - name: Test
+      run: npm run cypress:run
+
+    - name: Update summary
+      run: |
+        npx mochawesome-merge ./tests/cypress/reports/*.json -o tests/cypress/reports/mochawesome.json
+        rm -rf ./tests/cypress/reports/mochawesome-*.json
+        npx mochawesome-json-to-md -p ./tests/cypress/reports/mochawesome.json -o ./tests/cypress/reports/mochawesome.md
+        npx mochawesome-report-generator tests/cypress/reports/mochawesome.json -o tests/cypress/reports/
+        cat ./tests/cypress/reports/mochawesome.md >> $GITHUB_STEP_SUMMARY
+
+    - name: Upload artifacts
+      uses: actions/upload-artifact@v4
+      if: failure()
+      with:
+        name: cypress-artifact-mailchimp
+        retention-days: 7
+        path: |
+          ${{ github.workspace }}/tests/cypress/screenshots/
+          ${{ github.workspace }}/tests/cypress/videos/
+          ${{ github.workspace }}/tests/cypress/logs/
+          ${{ github.workspace }}/tests/cypress/reports/

.gitignore

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+node_modules
+.DS_Store
+.idea*
+vendor/*
+release/*
+dist/*
+docs-built
+.phpunit.result.cache
+/.wp-env.override.json
+
+tests/cypress/videos
+tests/cypress/screenshots
+tests/cypress/reports
+tests/cypress/downloads
+
+mailchimp.zip

.nvmrc

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+v20

.wp-env.json

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+{
+  "core": null,
+  "plugins": ["."],
+  "env": {
+    "tests": {
+      "mappings": {
+        "wp-cli.yml": "./tests/bin/wp-cli.yml"
+      }
+    }
+  },
+  "lifecycleScripts": {
+    "afterStart": "bash ./tests/bin/initialize.sh"
+  }
+}