Declare SymbolConstructor for older versions of ES

Older versions of the ES standard do not define `Symbol#asyncIterator`.
This means our attempts to "manually" define `AsyncIterator` for
versions of ES that don't have it fail.

Following [1] and [2], our understanding is that the best existing
solution is to manually re-define `SymbolConstructor` to have this
member.

[1]: microsoft/TypeScript#13031
[2]: microsoft/TypeScript#8099

Author: hausdorff

nodejs/query/interfaces.ts

@@ -22,6 +22,12 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
+declare global {
+    export interface SymbolConstructor {
+        readonly asyncIterator: symbol;
+    }
+}
+
 export interface AsyncIterator<T> {
     next(value?: any): Promise<IteratorResult<T>>;
     return?(value?: any): Promise<IteratorResult<T>>;