Add imports / exports lint rules

[SuperSodaSea]

See pixijs/pixijs#8932 for details.

ESLint plugin eslint-plugin-import and eslint-plugin-simple-import-sort are added.

The following ESLint rules are added or modified:

{
    "no-multi-spaces": [1, { "exceptions": {} }],
    "object-curly-newline": [1, { "multiline": true, "consistent": true }],

    "import/first": 1,
    "import/newline-after-import": 1,
    "simple-import-sort/imports": [1, {
        "groups": [
            [
                "^\\w", // External
                "^\\u0000", // External side effect
                "^@pixi/", // @pixi/*
                "^\\u0000@pixi/", // @pixi/* side effect
                "^\\.", // Relative
                "^\\u0000\\.", // Relative side effect
                "^" // Others
            ],
            [
                "^\\w.*\\u0000$", // External types
                "^@pixi/.*\\u0000$", // @pixi/* types
                "^\\..*\\u0000$", // Relative types
                "\\u0000$" // Other types
            ]
        ]
    }],
    "simple-import-sort/exports": 1
}

Also the indent of index.js is fixed (originally mixed 2 and 4 spaces, now 4 spaces).

[SuperSodaSea]

@bigtimebuddy By the way, do you think it's a good idea to apply the lint rules to @pixi/eslint-config itself? (Adding a .eslintrc.json that contains { "extends": ["./index.js"] })