Update WebAssembly declaration

TS 3.5 and above have WebAssembly types, so the ambient declaration in
declarations.d.ts needs to be compatible with that declaration. The
previous declaration, `declare const`, was not. This PR changes it to
`declare namespace`.

declarations.d.ts

@@ -255,7 +255,7 @@ declare const hotAddUpdateChunk;
 declare const parentHotUpdateCallback;
 declare const $hotChunkFilename$;
 declare const $hotMainFilename$;
-declare const WebAssembly;
+declare namespace WebAssembly {}
 declare const importScripts;
 declare const $crossOriginLoading$;
 declare const chunkId;