This PR:

1. Cleans up the implementation of Paths:

   • Removes certain unnecessary conditionals.
   • Improves the formatting a bit.
   • Improves comments

2. Adds WeakMap and WeakSet in the exclusion list.

   type Current = Paths<{a: WeakMap<{a: string}, number>; b: WeakSet<{a: string}>}>;
   //=> "a" | "b" | "a.delete" | "a.get" | "a.has" | "a.set" | "b.delete" | "b.has" | "b.add"
   
   type Updated = Paths<{a: WeakMap<{a: string}, number>; b: WeakSet<{a: string}>}>;
   //=> "a" | "b"

This PR is basically for the cleanup, it doesn't change any logic. The WeakMap / WeakSet fix is included here because it's a pretty minor fix.