Update: allow sync operation at root level (fixes: #7985)

[VictorHom]

What is the purpose of this pull request? (put an "X" next to item)

[X] Changes an existing rule (template)

What rule do you want to change?
This address issues #7985
This update lets the no-sync rule take an option parameter allowAtRootLevel: boolean that will allow sync operations at the root level.

Does this change cause the rule to produce more or fewer warnings?
less errors since it is making the rule more lenient.

How will the change be implemented? (New option, new default behavior, etc.)?
New option

Please provide some example code that this change will affect:

<!-- example code here -->
//no-sync: [1, { allowAtRootLevel: true }]

// good
const contents = fs.readFileSync('/super/important.txt')

// bad
function someFunction() {
    fs.readFileSync('file.txt'); // very clearly not root of module
}

What does the rule currently do for this code?
If sync operations are found, it reports

What will the rule do after it's changed?
if allowAtRootLevel:true is used, no errors will be reported for sync operations at the root level. If functions are at in function declaration/ function expressions and sync operations are inside, it will be reported

[mention-bot]

@VictorHom, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mduvall, @vitorbal and @btmills to be potential reviewers.

[eslintbot]

LGTM

[eslintbot]

LGTM

[eslintbot]

LGTM

[eslintbot]

LGTM

[not-an-aardvark]

Instead of manually keeping track of function scope, I think it would be simpler to use a descendant AST selector:

module.exports = {
    meta: {
        // ...
    },
    create(context) {
        const selector = context.options[0] && context.options[0].allowAtRootLevel
            ? ":function MemberExpression[property.name=/.*Sync$/]"
            : "MemberExpression[property.name=/.*Sync$/]"

        return {
            [selector](node) {
                context.report({
                    node,
                    message: "Unexpected sync method: '{{propertyName}}'.",
                    data: {
                        propertyName: node.property.name
                    }
                });
            }
        }
    }
};

That way you won't have to do any of the function scope tracking yourself.

[VictorHom]

that's what I wanted to do, but didn't know how to. Will know better next time!
@not-an-aardvark I fixed with your changes

[eslintbot]

LGTM

[platinumazure]

Looks great to me, thanks!

Unrelated to your PR, I'm wondering if we should add tests for chained calls off of a .*Sync function call (e.g. foo.somethingSync(). transform()). I'll try to hold onto that thought and maybe make a PR later.

[not-an-aardvark]

LGTM, thanks!