Add a rule option to only detect/fix top-level keys

[longzheng]

It would be great if we can specify an option so that only the top-level keys are sorted, while nested keys are ignored/left alone.

For example using the default options

/* eslint sort-keys/sort-keys-fix: "error" */

export const test = {
    b: 'b',
    a: {
        b: 'b',
        a: 'a',
    },
};

will be sorted to

export const test = {
    a: {
        a: 'a',
        b: 'b',
    },
    b: 'b',
};

It would be great if I can specify an option like nested: true

/* eslint sort-keys/sort-keys-fix: ["error", "asc", {nested: false}] */

export const test = {
    b: 'b',
    a: {
        b: 'b',
        a: 'a',
    },
};

so it becomes

export const test = {
    a: {
        b: 'b',
        a: 'a',
    },
    b: 'b',
};