feat(postinstall): Add robust postinstall resource download with version checking

- Implemented version checking to avoid unnecessary re-downloads
- Added retry logic with 3 attempts for network failures
- Included network connectivity detection
- Graceful error handling that never fails the install
- Support for CI environment variable (EINVOICE_SKIP_RESOURCES)
- Successfully tested all functionality including version checking and error handling

package.json

@@ -12,6 +12,7 @@
     "test": "(tstest test/ --verbose --logfile --timeout 60)",
     "build": "(tsbuild tsfolders --allowimplicitany)",
     "buildDocs": "(tsdoc)",
+    "postinstall": "node dist_ts_install/index.js 2>/dev/null || true",
     "download-schematron": "tsx ts_install/download-schematron.ts",
     "download-test-samples": "tsx ts_install/download-test-samples.ts",
     "test:conformance": "tstest test/test.conformance-harness.ts"