Disable `no-var` on `globals.d.ts`

To create globals available on `globalThis`, you must use `var`: https://stackoverflow.com/a/56984941/288906 What do you think about adding an exclusion for files named global.d.ts and globals.d.ts? A proper fix for this would have to use the TypeScript parser to determine when `var` is being used inside a `declare global` in any file, or on the top-level in environment types… but this means creating a whole new `no-var` rule. I think this included exception would be a good compromise to avoid so much confusion.