Add test coverage script (#929)

## Description:

Added a command "npm run test:coverage"
This could be added to the CI/CD to check if a pull request induced
regressions, and if it added propers tests.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

theodoreleon.aetarax

package.json

@@ -10,6 +10,7 @@
     "dev": "cross-env GAME_ENV=dev concurrently \"npm run start:client\" \"npm run start:server-dev\"",
     "tunnel": "npm run build-prod && npm run start:server",
     "test": "jest",
+    "test:coverage": "jest --coverage",
     "format": "prettier --ignore-unknown --write .",
     "lint": "eslint",
     "lint:fix": "eslint --fix",