Feat(package.json): add eslint-staged scripts

What does this MR do?

This MR adds optional eslint-staged and eslint-staged-fix scripts to the package.json, so that developers can add these to run in their pre-commit hooks. The main reason to add these scripts is for developers, who don't want to run a linter in their editor and run prettier on save, but rather have it as a pre-commit hook.

An example pre-commit script could look like this:

#!/bin/sh
set -e

yarn prettier-staged
yarn eslint-staged
yarn stylelint

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

This MR introduces little risk as these scripts have to be manually used for them to be active. It could be that some files are included erroneously, but even when added as a pre-commit hook, this can always be overridden with --no-verify and finally the build would still be the final source of truth if the MR should pass.

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team