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

[fregante]

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.