Add coverage folder to .gitignore

[juanmaguitar]

Description

The coverage folder is not included in the gutenberg repository but this folder is generated every time I run the tests locally and is being tracked by git so you have to be careful not to add this folder in commits.

In this PR I added the coverage folder to .gitignore file so is not being tracked by git anymore

How has this been tested?

By executing commands such as npm run test-unit and checking how the generated coverage folder is ignored by git when doing git status

Types of changes

Minor Git control change.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[gziolo]

@youknowriad any objections here? We had this folder ignored before, but it was removed together with the default handling for code coverage that we stopped using.

[youknowriad]

I don't mind, but if we can remove its generation entirely, maybe that's better.

[gziolo]

I don't think this folder is created by default when you run npm run test-unit, but you can always enable the coverage with a CLI option.

[youknowriad]

got it, let's add the ignore folder, it's fine.

[gziolo]

@juanmaguitar, thank you for the contribution 👍🏻