fix(create-react-native-library): ignore test app artifacts

[dcavalcante]

Problem

create-react-native-library generates the test app with react-native-test-app, then removes its generated .gitignore. That loses test-app-specific rules for generated local artifacts.

Fix

Preserve the generated .gitignore only for the test-app example. Other example flows retain their existing cleanup behavior and manage their own ignore rules where applicable.

Validation

• Generated the test-app from the pre-change and current code; only the current output retains example/.gitignore.
• Verified the retained file includes .xcode.env and *.xcworkspace/ rules.
• Exercised the non-test-app cleanup branch; it removes the upstream file and retains Expo's generated replacement rules.
• Ran ESLint, TypeScript build, and git diff --check.

[satya164]

these are ignored for all templates but only relevant for test app, and could be important for other templates. e.g. .xcode.env is intended for source control for regular react native apps.

a better approach maybe to keep the example's .gitignore instead of deleting it to keep example specific ignores.

[dcavalcante]

Updated to preserve react-native-test-app’s generated .gitignore instead of adding its rules to the shared template. Verified pre-change has no example/.gitignore; this change preserves the test-app rules, including .xcode.env and *.xcworkspace/