Create index.js file for main node exports.

Also fix up some of the values in package.json to reflect changes.

I think with this we may be ready to register `stringencoding` on npm. @inexorabletash would you like to do that?

I don't think exporting `encoding-indexes.js` like this will do much though since we would have to include them on TextEncoder/Decoder's global before we require them. I think the only way to get around that problem would be to add `nodejs` specific code to `encoding.js` to require it if it's in node and attach it to `this`. Another solution would be to change the library a little to be able to pass in a specific index to the decoder's or encoder's constructor.