Fix broken CI hang installing playwright/chromium.

[mattheu]

Summary

• Add postinstall script (tests/e2e/build-wp-utils.js) that compiles @wordpress/e2e-test-utils-playwright TypeScript source to CommonJS and patches the package's own exports to point at the compiled output — the package ships only TypeScript source which Node cannot load from node_modules
• Update dependency versions (@playwright/test, @wordpress/e2e-test-utils-playwright, @wordpress/env, @wordpress/scripts, @types/node)
• Convert fixtures.js to consistent CommonJS (module.exports) to match the existing CJS Playwright config
• Add timeout and Docker image caching to the CI workflow

Test plan

• npm ci && npm run build completes without errors
• npm run wp-env start && npm run test:e2e passes all tests locally
• CI workflow runs green on this PR

[github-actions]

Playwright test results

 2 failed
 16 passed

Details

 18 tests across 6 suites
 2 minutes, 27 seconds
 a6fa453

Failed tests

chromium › exclude-with-post-in.spec.js › Exclude Displayed Posts with post__in › should exclude displayed posts even when post__in is set via Advanced Query Loop
chromium › exclude-with-post-in.spec.js › Exclude Displayed Posts with post__in › should properly filter post__in array when multiple post templates used

[roborourke]

Relevant upstream tickets - this was a mistake in the release and should be getting fixed in a release soon if not already:

• @wordpress/e2e-test-utils-playwright gives error "Stripping types is currently unsupported for files under node_modules" WordPress/gutenberg#79018
• e2e-test-utils-playwright: start transpiling again, but faster WordPress/gutenberg#79026

[mattheu]

Superseded by #24