add-coverage-to-gitignore (#35809)

Summary:
There is a large amount of autogenerated files in the coverage folder when running `jest --coverage`.

This folder is also in .gitignore for react projects created with `create-react-app`

Originally suggested here: react-native-community/discussions-and-proposals#569

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[GENERAL] [ADDED] - Added "coverage" folder generated på `jest --coverage` to .gitignore

Pull Request resolved: #35809

Test Plan:
1. cd template
2. yarn
3. yarn test --coverage

Im not sure why many of the files in the template folder have an underscore(_) in the filenames instead of a dot(.). When creating a react-native project they have a dot. So if you run the above commands, the coverage folder will not be ignored since its not in `.gitingore` but in `_gitignore`. However, if you temporarily create a `.gitignore` file, you will see that the coverage folder is ignored.

4. Temporarily create `.gitignore` file to verify that coverage folder is ignored

Reviewed By: cortinico

Differential Revision: D42454560

Pulled By: jacdebug

fbshipit-source-id: 0be007d3ba3dde47814c0ce0fb8b0ff9f67d637b

Author: Adnan-Bacic

template/_gitignore

@@ -61,3 +61,6 @@ yarn-error.log
 
 # Temporary files created by Metro to check the health of the file watcher
 .metro-health-check*
+
+# testing
+/coverage