* feat(staticfile): add configurable index fallback for SPAs
- Fixes #https://github.com/railwayapp/railpack/issues/533
- Introduce index_fallback option in Staticfile config to toggle SPA routing.
- Improve root directory detection by trimming whitespace from environment variables and configuration.
* feat: add index fallback example for static files
- Create a new example directory demonstrating the index_fallback configuration.
- Include a test case to verify that non-existent routes correctly serve index.html.
Generated-by: aiautocommit
* test: update snapshots
Generated-by: aiautocommit
* test: update static file config and index test cases
Generated-by: aiautocommit
* test: add SPA index fallback verification to test suite
Generated-by: aiautocommit
* docs: document RAILPACK_VERBOSE and clarify Staticfile config
Generated-by: aiautocommit
