style(lib/checks): var -> let and const codemod

[gaiety-deque]

This is a piece of a larger creative time PR so it's easier to review, the full scope can be seen here and includes eslint adjustments not seen here https://github.com/dequelabs/axe-core/pull/4444

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Who else does this?

• Google (JS style guide) https://google.github.io/styleguide/jsguide.html#features-local-variable-declarations
• Airbnb (popular JS style guide) https://github.com/airbnb/javascript

Why do this?

Gaiety's explanation: "let works the way most devs think var works. scope leaking and variable hoisting is a messy confusing idea, JS got updated for a reason" also it's 2024, this isn't too controversial anymore

• https://www.freecodecamp.org/news/var-let-and-const-whats-the-difference/
• https://blog.javascripttoday.com/blog/var-vs-let-vs-const/ "The var keyword was originally used for declaring variables in JavaScript. If you’ve browsed an old codebase or an older JavaScript tutorial, you have probably seen it."
• https://medium.com/@rithinmenezes/understanding-let-var-and-const-in-javascript-a-guide-to-variable-declarations-f271c5b8dc79
• Airbnb's why https://airbnb.io/javascript/#variables

[romankulkovsf]

Thnks