Chore: prefer smaller scope for variables in codebase

[not-an-aardvark]

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

[x] Other, please explain:

What changes did you make? (Give an overview)

I made an attempt at implementing the prefer-smaller-scope rule proposal, which was originally proposed in #5284. That attempt can be found here.

Unfortunately, I found that the rule would incorrectly report some places where declaring a variable in an outer scope is necessary if an inner scope is executed multiple times. I don't know of a good way to avoid those false positives while still keeping the rule useful.

let flag; // `flag` is only ever used in the function, but still needs to be declared outside it.

function doSomethingAfterFirstCall() {
    if (flag) foo();
    flag = flag || true;
}

However, by implementing the rule I found a number of cases where we can reduce the scope of the variables.

Is there anything you'd like reviewers to focus on?

Nothing in particular

[eslintbot]

LGTM

[mention-bot]

@not-an-aardvark, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mysticatea, @gyandeeps and @IanVS to be potential reviewers.