Option to rename MainModule for generated TypeScript types

[Mike-Bell]

Looks like this is hardcoded to output MainModule

emscripten/src/embind/embind_gen.js

Line 351 in e306209

out.push('export interface MainModule {\n');

I can alias it on the consuming side pretty easily, but it would be nice if this were configurable, as MainModule is not a great name and it will conflict between multiple modules.