AMP Stories: Use Gutenberg ESLint config

[swissspidy]

While working on #1885 and #1893 I noticed some odd ESLint rules that I thought could be streamlined so they're more in line with PHP and with core.

When looking into it, I found #1019 with relevant commits 277da63 and 34024cc.

I noticed that eslint-config-wordpress is being used as the foundation for the ESLint config, which hasn't seen any new notable updates since 2017. However, a lot has changed since then.

The WordPress JS coding standards have evolved over time, mainly due to Gutenberg development. Gutenberg has way more rules for JS development in WordPress. Some of them which were manually ported over into this repository.

Nowadays, Gutenberg uses and publishes @wordpress/eslint-plugin as a central way to get an up-to-date ESLint config.

For this PR I adapted the config and then tried to fix all linting errors. The diff is quite long, mainly due to the following two reasons:

• Trailing commas for objects and arrays (like in PHP)
• Variables that don't change are now const, and not just let or even var.

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I think this could be a good starting point to improve the code base in the long term, separate components, and make them testable.

For example, I am thinking about adopting @wordpress/scripts for testing and configuration. Thanks to WordPress/gutenberg#13814 we can simplify #1886 even further.