rustdoc: replace `var` with `const` and `let` in JS

[jsha]

Now that rustdoc is using ES6 idioms, we should replace all var declarations with const and let. This style guideline explains why: https://github.com/airbnb/javascript#references. Once that's done, we should turn on the lint checks that enforce it.

I think there are a few places (at least in search.js) where we rely on the surprising property of var declarations called "var hoisting". In those cases we may to move some declarations to the top of their function at the same time we change them to let.

[GuillaumeGomez]

I'll do it once I'm done with #90630.

[the8472]

Shouldn't the JS also be changed to strict mode to ensure that all variables actually are declared via keywords?

[GuillaumeGomez]

@Folyd: So a few things remain to be done:

• Move callback to the () => {} syntax.
• Use "strict" as suggested by @the8472
• Add a lot more eslint lints.

Do you want to divide the work with me? :)

EDIT: sorry @fmease for the invalid ping ^^'

[Folyd]

@Folyd: So a few things remain to be done:

• Move callback to the () => {} syntax.
• Use "strict" as suggested by @the8472
• Add a lot more eslint lints.

Do you want to divide the work with me? :)

EDIT: sorry @fmease for the invalid ping ^^'

Sure, I'd love to. 😸