Add es2018.intl ref to es2020.intl (#49152)

* Add es2018.intl ref to es2020.intl

es2020.intl refers to NumberFormatPartTypes declared in es2018.intl
as of #46508.

I'm not sure how to test this; it repros on Definitely Typed in
types/ndarray, but when I copy the same files into a compiler test it
passes without a problem.

* Add a test that shows the change works

It doesn't actually show that the original bug has been fixed,
though.

src/lib/es2020.intl.d.ts

@@ -1,3 +1,4 @@
+/// <reference lib="es2018.intl" />
 declare namespace Intl {
 
     /**