chore: add ESLint config file and deps

[Soare-Robert-Daniel]

Summary

• Install WordPress eslint plugin and eslint using --legacy-peer-deps to avoid conflicts with wp-scripts
• Add EsLint config file in which non-essential rules are marked as warn instead of error.
• Small clean-up for some rules.
• Enabling the lint check on GA

Will affect visual aspect of the product

NO

Screenshots

Test instructions

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Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #978

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🎉 This PR is included in version 5.0.0 🎉

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