element: move the `element.js` file to the root

Other plugins are doing `define('scribe-common/element')`, so for this
to work in npm/browserify then we need this file to be located at the
root of the repository.
package: set "main" to element.js

There is no `scribe-common.js` file, and `element.js` appears
to be the only JS file in the repo at the moment.