Update assets compilation to properly use Babel transpiling + shims

Previously, we were using mix.babel() to compile JS files. As this runs the files through a Babel engine and concats, but doesn't properly shim the code for ES5. The JS command will transpile with Babel, pull in proper configs, and shim the code as well.

Signed-off-by: Martin Lowe martin.lowe@eclipse-foundation.org