Add codegen script generated files to .gitignore (#26479)

Summary:
The files autogenerated by `scripts/generate-rncore.sh` aren't to be committed. This pull request adds them to the repo's `.gitignore`

## Changelog

[Internal] [Changed] - Add codegen generated files to gitignore
Pull Request resolved: #26479

Test Plan: The files generated by `generate-rncore.sh` are no longer added when you run `git add .`

Differential Revision: D17439794

Pulled By: TheSavior

fbshipit-source-id: 54a2e07aefc5fd187ba5eb8de0c2cd8b69ee4053

Author: empyrical

.gitignore

@@ -82,3 +82,7 @@ RNTester/build
 /template/ios/Podfile.lock
 /RNTester/Pods/
 /RNTester/Gemfile.lock
+
+# react-native-codegen
+/ReactCommon/fabric/components/rncore/
+/schema-rncore.json