Fix detecting an existing Map/Set

[elibarzilay]

This didn't affect compilation to CJS since that sets exports.Map
instead of creating a global.

[rbuckton]

The point of the declare const Map on the preceding line is to force the lookup of Map to look at the globally scoped Map (rather than ts.Map due to namespace merging). How did this become an error for CJS emit?

[elibarzilay]

The problem is not in the CJS output. If I set module to es6, the resulting code is:

var NativeCollections;
(function (NativeCollections) {
    function tryGetNativeMap() {
        return typeof Map !== "undefined" && ...;
    }
    ...
})(NativeCollections || (NativeCollections = {}));
export var Map = getCollectionImplementation("Map", "tryGetNativeMap", "createMapShim");
```

[elibarzilay]

@rbuckton, does that make sense?

[rbuckton]

There's a few more small issues I noticed, after which this should be ready to merge.

[rbuckton]

Minor formatting nit:

Suggested change

	const globals = typeof globalThis !== "undefined" ? globalThis
	: typeof global !== "undefined" ? global
	: typeof self !== "undefined" ? self
	: undefined;
	const globals = typeof globalThis !== "undefined" ? globalThis :
	typeof global !== "undefined" ? global :
	typeof self !== "undefined" ? self :
	undefined;

[rbuckton]

globals is possibly undefined.

Suggested change

	const gMap = globals.Map;
	const gMap = globals?.Map;

[rbuckton]

globals is possibly undefined.

Suggested change

	const gSet = globals.Set;
	const gSet = globals?.Set;