feat: update one-var for class static blocks

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

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

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[x] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Refs #15016, fixes one-var.

What changes did you make? (Give an overview)

This PR fixes false positives of the one-var rule with option "always" where the rule suggests combining declarations from a class static block with declarations from the upper scope.

/* eslint one-var: ["error", "always"] */

let a;

class C {
    static {
        let b; // false positive
    }
}

Also enables autofixing declarations at the top level of class static blocks with option "never".

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

Documentation for this rule states that it applies to functions, but it actually applies everywhere: at the top level, in functions, and in all blocks for block-scoped variables.

[nzakas]

LGTM