suggestion: deprecate `std/collections` APIs in favour of new `Set` methods

I suggest the following now that new `Set` methods are [available in Deno](https://deno.com/blog/v1.42#v8-123-and-typescript-543):
1. Deprecate [`intersect()`](https://jsr.io/@std/collections/doc/intersect/~/intersect) in favour of [`Set.prototype.intersection()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/intersection)
2. Deprecate [`union()`](https://jsr.io/@std/collections/doc/union/~/union) in favour of [`Set.prototype.union()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/union)

Removals should happen in `std/collections@1`. However, this might be too soon as these APIs are not fully supported in other JavaScript runtimes.