Implement a JavaScript is_enabled() function

[peterhartree]

Often feature changes involve PHP and JavaScript changes. It'd be useful to be able to call is_enabled('flag_key') via JavaScript as well.

[jamesrwilliams]

This is a great idea. Something like this perhapse:

if ( featureFlags.is_enabled( 'foo') ) { /* Feature specific JS */ } 

[jamesrwilliams]

Just making a note that this should inherit the function name we decide to implement/update in #15.

[jamesrwilliams]

Potentially a separate issue but we should also consider how this can be used within WordPress's new Gutenberg blocks.