Drop all those unnecessary polyfills

[Sukka]

https://github.com/un-es/eslint-plugin-i/blob/616ac75b11731f41a3786bdbe2c7620d65df9ded/package.json#L102-L118

There are WAY TOO MANY polyfills in the dependencies. Let's target Node.js 14 and drop all of them!

[Sukka]

According to https://node.green, even Node.js 12 supports all Array.prototype.includes, Array.prototype.flat, Array.prorotype.flatMap, and Object.values. That's to say, we can drop all of the polyfills if we target Node.js 12.