update lint-staged to v10 for consistency

[Belco90]

Last version available of lint-staged is v10. On that major release is not necessary anymore to add the "git add" step, and actually has to be removed in case you got it from a previous version.

The course repo itself is fine, as it's using v9 with "git add" step. But then when someone following proper lesson on Testing Javascript installs lint-staged dependency, they'll get v10 and the config steps are not consistent anymore, which could cause them having issues with pre-commit hook.

This PR is updating lint-staged and its config to mirror what people following the course will see, but there are 2 things that must be updated additionally:

• Auto-format All Files and Validate Relevant Files in a precommit Script with lint-staged lesson to remove any reference to "git add" step as it's not necessary anymore
• probably rebase lesson branches to get this change

[kentcdodds]

Thank you 👍

[kentcdodds]

I'll update the videos at a later time.