Improve ES6 support in TypeScript definitions generation

Users building with --emit-tsd and the EXPORT_ES6 setting should get
TypeScript definitions accounting for the fact that ES6 modules
include a default export that is a functor to instantiate and access
the wasm module in question

When generating TS definitions, query the EXPORT_ES6 setting and,
if present, add the appropriate exports to the output

Include options param in default functor export for `--emit-tsd`.
This serves as a starting point for incluing all options such as
`locateFile` (and the slew of others), without requiring manually
iterating through them

Update tests to include factory function

Author: jnickg

test/other/embind_tsgen_ignore_2.d.ts

@@ -109,3 +109,4 @@ interface EmbindModule {
   wstring_test(_0: string): string;
 }
 export type MainModule = WasmModule & EmbindModule;
+export default function MainModuleFactory (options?: unknown): Promise<MainModule>;

tools/emscripten.py

@@ -631,6 +631,8 @@ def create_tsd(metadata, embind_tsd):
   if embind_tsd:
     export_interfaces += ' & EmbindModule'
   out += f'export type MainModule = {export_interfaces};\n'
+  if settings.EXPORT_ES6 and settings.MODULARIZE:
+    out += 'export default function MainModuleFactory (options?: unknown): Promise<MainModule>;\n'
   return out