Make @typescript-eslint optional peerDependencies

Created by: ThewBear

Problem

Currently eslint-config-react-app requires @typescript-eslint to be installed in every project even the project that doesn't use typescript. This lead to the warning of missing typescript (#6834 , #6859).

Solution

By making @typescript-eslint an optional peerDependencies, non-typescript user doesn't need to installed @typescript-eslint and can get rid of the warning. User who use typescript can installed typescript along with @typescript-eslint/eslint-plugin and @typescript-eslint/parser.