Support for transforming .js files

[dmorejon]

We would like to add support for transforming Javascript files in a directory, usually helper scripts, in addition to the .html tests.
There should be very little work here, since the code already transforms scripts embedded in HTML. The only exception might be looking into imports.

If this is done, it would make sense to change the wrapper code in index.js for directory transformations to do 2 passes: first pass is for all the helper .js scripts (these should be roughly independent), then another pass to transform the .html tests and run verification. The reason it is best to transform the .html second is many of the tests use the helper scripts, and if those are not transformed the run_web_tests.py framework will flag the functions as errors since it doesn't recognize the js-test.js functions.