Split ESLint config package into multiple modular packages

[karlhorky]

Instead of providing one monolithic package that has all of our configuration inside (JavaScript, Node.js, React, Next.js, PostgreSQL), it would be nice to be able to only install the configuration and dependencies that a certain student project absolutely needs.

The ESLint configuration option extends receives an array of configurations, which we can add multiple items to:

{
    "extends": [
        "upleveled-js",
        "upleveled-react",
        "upleveled-next-js",
        "upleveled-postgres-js",
    ],

Important: Each of these separate packages should not have rules or plugins which overlap or conflict.