Added explicit extension for imported .wasm file for --target bundler

[ibaryshnikov]

Closes #1441

This update is more in line with esm-module integration. At the moment there's a comment near this code:

// With Bundlers and modern ES6 support in Node we can simply import
// the wasm file as if it were an ES module and let the
// bundler/runtime take care of it.

If I get it right, without extension this will not work in node.js

[alexcrichton]

Looks good to me, thanks!

I don't think this works for some more esoteric use cases but since this is scoped to just bundlers I think it should work out alright.