cli: support '--example=empty' to create empty app

[eandre]

To support scripting without the interactive template picker,
add --example=empty as a way to create an empty app.

Thanks Ismoil Hasanov for the suggestion.

[encore-cla]

All committers have signed the CLA.

[github-actions]

Test Results

    2 files  +       1  219 suites  +64   5m 33s ⏱️ - 12m 5s
719 tests +   187  716 ✔️ +   195  3 💤 +3  0 ❌  - 3 
719 runs   - 1 873  716 ✔️  - 1 865  3 💤 +3  0 ❌  - 3 

Results for commit 1b07198. ± Comparison against base commit 0be2cc4.

This pull request removes 15 and adds 202 tests. Note that renamed tests count towards both.

encr.dev/e2e-tests ‑ TestRun/encore_currentrequest
encr.dev/e2e-tests ‑ TestRun/et_override_user
encr.dev/e2e-tests ‑ TestRun/et_override_user_authdata
encr.dev/e2e-tests ‑ TestRun/experiment_local_secrets_override
encr.dev/e2e-tests ‑ TestRun/fallback_routes
encr.dev/e2e-tests ‑ TestRun/pubsub_method_handler
encr.dev/e2e-tests ‑ TestRun/pubsub_ref
test/svc ‑ TestRun
test/svc ‑ TestRun/encore_currentrequest
test/svc ‑ TestRun/et_override_user
…

encore.dev/appruntime/apisdk/api ‑ TestDescGeneratesTrace
encore.dev/appruntime/apisdk/api ‑ TestDescGeneratesTrace/echo
encore.dev/appruntime/apisdk/api ‑ TestDescGeneratesTrace/invalid
encore.dev/appruntime/apisdk/api ‑ TestDescGeneratesTrace/raw
encore.dev/appruntime/apisdk/api ‑ TestDescGeneratesTrace/unauthenticated
encore.dev/appruntime/apisdk/api ‑ TestDesc_EndToEnd
encore.dev/appruntime/apisdk/api ‑ TestDesc_EndToEnd/echo
encore.dev/appruntime/apisdk/api ‑ TestDesc_EndToEnd/invalid
encore.dev/appruntime/apisdk/api ‑ TestDesc_EndToEnd/unauthenticated
encore.dev/appruntime/apisdk/api ‑ TestRawEndpointOverflow
…