Lib generator needs work to support TS 5.6

[uhyo]

TS 5.6's lib.esnext.iterator.d.ts introduces a new way of defining type defintions (microsoft/TypeScript#58222):

The lib .d.ts file itself is a module (due to existence of export {};) and uses a declare global block to define some premordial types.

Our lib generator needs to support this construct too. (and yes, there are several any in that file which I really want to alter!)