Rework jsv.js export/require shim to match uri.js

[nickcmaynard]

Webpack doesn't detect the require() statements in jsv.js as-is - I tracked this down to the exports re-definition. The form used in uri.js works correctly in both Webpack and all the test pages, so I brought that into jsv.js.

So this pull request is a simple copy of the export/require definition logic from uri.js to jsv.js.